fix test parseManyAliasesForCollections

[aSemy]

I think the test is failing on CI because the test is performing worse than expected.

Example failure:

ReferencesTest[jvm] > Parsing with aliases may take a lot of time, CPU and memory[jvm] FAILED
    org.opentest4j.AssertionFailedError: Time was 11.039 seconds. ==> expected: <true> but was: <false>
        at app//org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
        at app//org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
        at app//org.junit.jupiter.api.AssertTrue.failNotTrue(AssertTrue.java:63)
        at app//org.junit.jupiter.api.AssertTrue.assertTrue(AssertTrue.java:36)
        at app//org.junit.jupiter.api.Assertions.assertTrue(Assertions.java:214)
        at app//org.snakeyaml.engine.usecases.references.ReferencesTest.parseManyAliasesForCollections(ReferencesTest.java:113)

• Remove the num-of-cores logic regarding minDuration - 400ms seems fine.
• Increase max duration to 15 seconds - the exact timeout doesn't particularly matter, this test is always going to be slow.
• replace manual time measurement with Java 8 time utils
• add a JUnit timeout assertion