Closes #46431

The various reading methods when resolving keys can potentially return
null. The rule was as follows:

1. If the thing being written can never be null, I put a ! after the
   Read call.
2. If the thing being written could be null, no suppression was
   allowed.

Essentially this means that the reading code is not expected to be
robust in the face of nulls coming from incorrectly annotated code.

Some of the types (like SymbolKeyReader) are pooled. I elected to
leave the members annotated as they act when being used normally, and
suppressed null assignments when the object was in the pool.

We had left this as a legacy mode for users who were running into
issues. We recently reset the option for anybody using 16.7 builds
after fixing some underlying issues, and we haven't heard any new
issues.

Closes #46319

Fixes #46463

* Add ServiceLayer.Test that overrides any other layer or workspace kind.

Mark all services used for testing with ServiceLayer.Test.
Remove WorkspaceKind.Test and set the kind of TestWorkspace to `host`.

* Move MockWorkspaceEventListenerProvider to Fatures test composition

* Enable solution crawler in diagnostic tests

DiagnosticEventTaskScheduler was only used in TaskQueue scenarios, and
the sequencing performed by the queue ensured that operations were only
scheduled in sequence to the task scheduler (i.e. one at a time). The
custom scheduler can simply be removed.

* Test compositions

* Eliminate TestExportProvider

* Fixes and caching

* Debugger proxies

* Scopes

* Feedback and more fixes

* RemoteHostOptions

* Throw without timeout on invalid thread switches

* EventCollectorTests requires a main thread

Bump versions of System.Collections.Immutable and System.Reflection.Metadata to consume the new function pointer metadata changes.