Improve parallel test execution

[marcphilipp]

Parallel test execution can reduce feedback time for developers considerably and thereby increase their productivity. While this is a popular feature, a number of issues have piled up that we should resolve to remove potential pitfalls and rough edges.

Deliverables

• Support some level of parallelization in junit-vintage-engine #2229
• Implement Parallel Method Execution in JUnit-Vintage engine #4238
• Add a way to inherit ResourceLocks that allows for more parallel execution of tests #3102
• Execution mode is needlessly restrictive for containers without execution behavior but with isolated children #2982
• Introduce support for virtual threads #3442
• JUnit parallel executor running too many tests with a fixed policy #3108