Build Service for restricting only one test task to execute at a time

build.gradle

import org.gradle.api.services.*

subprojects {
    apply plugin: OneTestTaskOnly
}

class OneTestTaskOnly implements Plugin<Project> {
    void apply(Project project) {
        // Register a service to constrain parallelism of test tasks to 1
        Provider<SharedResource> testLimitingService = project.gradle.sharedServices.registerIfAbsent("testLimitingService", SharedResource) { spec ->
            maxParallelUsages.set(1)
        }

        project.tasks.withType(Test).configureEach {
            // Tell gradle that test tasks require the testLimitingService to execute 
            usesService(testLimitingService)
        }
    }
}

// Since we are only constraining task parallelism, our service implementation is pretty bare
abstract class SharedResource implements BuildService<BuildServiceParameters.None> {
}