Add command line parameter that turns on leak tracking.

[polina-c]

CUJs that should be enabled:

1. Manual run of tests with CLI
2. Manual run of tests with IDE
3. Enable PR label that will turn on leak tracking just for PR that has it.  #138742
4. Enable regular validation for leaks in Flutter Framework. #138741

[flutter-dashboard]

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If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[jacob314]

[polina-c]

@godofredoc, can you review to make sure it is a right way to parameterize tests, to enable (1) use cases we discussed on Friday and (2) users to run with and without leak tracking?

[polina-c]

@keyonghan

[keyonghan]

The env variable name LGTM.