When running this against the HttpServer in the MP Rest Client 1.3 TCK, I get the following exception:

TestSSLwithStores.java

import java.io.*;
import java.net.*;
import java.security.*;
import javax.net.ssl.*;

public class TestSSLwithStores {

    private static String TEST_URL = "https://localhost:8948";
    private static String SSL_DIR = "/path/to/microprofile-rest-client/tck/src/main/resources/ssl/";

    public static void main(String[] args) throws Throwable {
        InputStream keyStream = new FileInputStream(SSL_DIR + "client.keystore");
        InputStream trustStream = new FileInputStream(SSL_DIR + "client.truststore");
        char[] password = "password".toCharArray();

        KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
        keyStore.load(keyStream, password);

        KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType());
        trustStore.load(trustStream, password);

        KeyManagerFactory keyFactory =
            KeyManagerFactory.getInstance(KeyManagerFactory.getDefaultAlgorithm());
        keyFactory.init(keyStore, password);
        KeyManager[] keyManagers = keyFactory.getKeyManagers();

        TrustManagerFactory trustFactory =
            TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
        trustFactory.init(trustStore);
        TrustManager[] trustManagers = trustFactory.getTrustManagers();

        SSLContext sslContext = SSLContext.getInstance("SSL");
        sslContext.init(keyManagers, trustManagers, null);
        SSLContext.setDefault(sslContext);

        URL url = new URL(TEST_URL);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.connect();
        System.out.println(connection.getResponseCode());

    }
}

I built the TestSSLWithStores class using guidance from: https://stackoverflow.com/questions/8339200/how-can-i-use-certificate-authentication-with-httpsurlconnection

Here is the exception I see when running this agains the HttpsServer in the MP Rest Client 1.3 TCK:

Exception in thread "main" javax.net.ssl.SSLException: readHandshakeRecord
	at java.base/sun.security.ssl.SSLSocketImpl.readHandshakeRecord(SSLSocketImpl.java:1072)
	at java.base/sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:402)
	at java.base/sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:567)
	at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:185)
	at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.connect(HttpsURLConnectionImpl.java:163)
	at TestSSLwithStores.main(TestSSLwithStores.java:38)
Caused by: java.net.SocketException: Broken pipe (Write failed)
	at java.base/java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:110)
	at java.base/java.net.SocketOutputStream.write(SocketOutputStream.java:150)
	at java.base/sun.security.ssl.SSLSocketOutputRecord.flush(SSLSocketOutputRecord.java:251)
	at java.base/sun.security.ssl.HandshakeOutStream.flush(HandshakeOutStream.java:89)
	at java.base/sun.security.ssl.Finished$T13FinishedProducer.onProduceFinished(Finished.java:664)
	at java.base/sun.security.ssl.Finished$T13FinishedProducer.produce(Finished.java:643)
	at java.base/sun.security.ssl.SSLHandshake.produce(SSLHandshake.java:436)
	at java.base/sun.security.ssl.Finished$T13FinishedConsumer.onConsumeFinished(Finished.java:969)
	at java.base/sun.security.ssl.Finished$T13FinishedConsumer.consume(Finished.java:848)
	at java.base/sun.security.ssl.SSLHandshake.consume(SSLHandshake.java:392)
	at java.base/sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:444)
	at java.base/sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:421)
	at java.base/sun.security.ssl.TransportContext.dispatch(TransportContext.java:178)
	at java.base/sun.security.ssl.SSLTransport.decode(SSLTransport.java:164)
	at java.base/sun.security.ssl.SSLSocketImpl.decode(SSLSocketImpl.java:1152)
	at java.base/sun.security.ssl.SSLSocketImpl.readHandshakeRecord(SSLSocketImpl.java:1063)
	... 5 more

When I test this with Apache CXF, I see these tests fail with essentially the same stack trace:

[INFO] Results:
[INFO] 
[ERROR] Failures: 
[ERROR]   SslContextTest>Arquillian.run:138->shouldSucceedMutualSslWithValidSslContext:65 » Processing
[ERROR]   SslMutualTest>Arquillian.run:138->shouldWorkWithClientSignature:121 » Processing
[ERROR]   SslMutualTest>Arquillian.run:138->shouldWorkWithClientSignatureCDI:143 » Processing
[ERROR]   SslMutualTest>Arquillian.run:138->shouldWorkWithClientSignatureFromClasspathCDI:148 » Processing

Note that the line number may be off somewhat, as I added the following code to the tests to allow me time to try running the standalone test case above:

System.out.println("ANDY: try it now: https://localhost:8948/");
        try {
            Thread.sleep(60000);
        }
        catch (Throwable t) {
            t.printStackTrace();
        }