Using pre_http_request filter to mock HTTP request responses in WordPress phpunit test https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/class-wp-http.php#L240-L262

filter-wp-request.php

<?php

class Test extends TestCase {

	public function test_function_that_makes_api_request(){
		add_filter('pre_http_request', function(){
			return [
				'body' => [
					'id' => 1,
				],
				'headers' => [
					'content-type' => 'application/json',
				],
				'response' => [
					'code' => 201,
				],

			];
		});

		$saved = SomeClass::saveAndPingApi();
		$this->assertTrue( $saved );

	}


	public function test_function_that_makes_api_request_with_error(){
		add_filter('pre_http_request', function(){
			return [
				'body' => [
					'error' => 'invalid request',
				],
				'headers' => [
					'content-type' => 'application/json',
				],
					'response' => [
					'code' => 400,
				],

			];
		});
		$saved = SomeClass::saveAndPingApi();
		$this->assertFalse( $saved );
	}

	
}