CQ. Consolidate context root discovery tests.

Move context root discovery coverage into AnalysisContextCollectionTest
so that discovery is validated through the collection that consumes its
results. Replace individual ContextRoot assertions with unified
expectations covering contexts, analysis options, workspaces, packages,
and analyzed files.

Verify that ContextRoot.isAnalyzed agrees with analyzedFiles for
existing, linked, and explicitly included paths. Register the new
expectation helpers and remove the obsolete ContextLocatorImpl test
suite.

Change-Id: Id79d248cd03be5a96d2ff51d3bfb4bd74b9dd2fb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/542981
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>