fix active editor files sometimes got attached to the wrong workspace folder

[connectdotz]

Fixed a bug that the active editor file might get attached to a wrong workspace folder (sometimes, deferred mode, race condition etc), even when the file is not under that workspace folder.

This could explain #1086, specifically in comment.

Also refactored the TestResultProvider.isTestFile to return a deterministic boolean value by parsing the content instead of returning a "maybe" value originally.

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Resolve #1086

[coveralls]

Pull Request Test Coverage Report for Build 7777442173

• 0 of 27 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 97.946%

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Totals
Change from base Build 7516526780:	0.001%
Covered Lines:	3917
Relevant Lines:	3925

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💛 - Coveralls