Test infrastructure for scope completion

Adds a new marker /*_*/ to trigger scope completion test.
Original type completion test oracles update for the tweaked output
Adds test cases for scope completion
SI-7280 Scope completion not returning members provided by imports

Updates localeContext() to return the best context possible when there are none directly
associated with the given position. It happens when an expression cannot be
successfully typed, as no precise ContextTree covers the expression location, or if the
position is not inside any expression.
Adds corresponding tests
Merge pull request #3140 from skyluc/issue/completion-import-object-7280

SI-7280 Scope completion not returning members provided by imports