[nomaster] Test infrastructure for scope completion

Adds a new marker /*_*/ to trigger scope completion test.
Original type completion test oracles update for the tweaked output
(cherry picked from commit 9c7c66ff7907e3ab814f0f4375eeaf6cdd230d5e)
[nomaster] Adds test cases for scope completion

(cherry picked from commit 3d55fe723f1af91f4d2db421f0e0965c583346dc)
[nomaster] 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
(cherry picked from commit 3028327e2a2b553b12ee45519413515c8aa0865f)