[New Feature]: Add tests that run only on demand

[heshanpadmasiri]

Description

Make it possible to annotate tests as "optional" so that they run only with a special flag to the bal test command.

Describe your problem(s)

While it is possible to disable a set of tests using --disable-groups, for projects that have tests that are not supposed to run by default (such as property-based tests that randomly generate a large number of random tests) this means their default test command becomes bal test --disable-groups <...> and their special case (running all tests) bal test. I think it is better if it is possible to make bal test the default option even for such cases.

Describe your solution(s)

Have an annotation (similar to enabled) that mark these tests run only on demand (say optional) and have a special flag (similar to --groups, say run --run-optional) that will run all of these tests as well. (Or make the --groups causes optional to be ignored)

Related area

-> Test Framework

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