Clarify test descriptor implementation requirements

[mpkorstanje]

• Define test and container as non-exclusive properties.
• Use emphasis to distinguish between term and word.
• Document that TestDescriptor.Type values are exclusive properties.
• Document that isContainer() and isTest() must be consistent with getType().
• Document that getChildren(), getDescendants() and mayRegisterTests() must be consistent with isContainer().
• Document that only the hierarchy of the test descriptor is mutable after discovery.

Fixes: #5168

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[mpkorstanje]

Draft for the purpose of team discussion.

Open questions:

• Require isContainer() as a precondition to calling AbstractTestDescriptor.addChild?
• Document that isTest, isContainer and getType should not be changed after discovery is complete? Does this apply to other properties too? If so, which?

[mpkorstanje]

Resolution after team discussion:

• Don't require isContainer() as a precondition to calling AbstractTestDescriptor.addChild.
• Document that only the hierarchy of the test descriptor is mutable after discovery.