flaky ParallelTestActionBuilderTest

on the current `4.0.0-M2` the `ParallelTestActionBuilderTest.testParallelBuilderNestedContainer` test is flaky! sometimes it passes, sometimes it fails. always with the following error message:

```shell
Error: Tests run: 99, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 12.032 s <<< FAILURE! - in TestSuite
Error: testParallelBuilderNestedContainer(org.citrusframework.actions.dsl.ParallelTestActionBuilderTest) Time elapsed: 0.212 s <<< FAILURE!
java.lang.AssertionError: expected [class org.citrusframework.actions.EchoAction] but found [class org.citrusframework.container.Sequence]
at org.citrusframework.actions.dsl.ParallelTestActionBuilderTest.testParallelBuilderNestedContainer(ParallelTestActionBuilderTest.java:79)
```

taken from [an example build](https://github.com/citrusframework/citrus/actions/runs/6504166883/job/17665724956) that passed on retry.

Start with org.citrusframework.actions.dsl.ParallelTestActionBuilderTest.testParallelBuilderNestedContainer and inspect the assertion at line 79. Run the test repeatedly, focusing on the reported mismatch between EchoAction and Sequence; done means the test no longer fails intermittently and the relevant test suite passes consistently.

Written by the indexing model from the issue text.