Cleaning up the Roslyn build configurations

Better codegen quality when emitting degenerate switches.

Related to dotnet/coreclr#7914
Where the offending pattern was observed in the IL generated for the Kestrel web server and was recommended as an easy improvement.

Becasue of cascaded dispatching into "try" regions , it is not uncommon to see async state machine to contain degenerate switches like

IL_0008:  switch     (
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_0729)

Note numerous cases all leading to the same target.
We can trivially emit such switches as just a range-check (that switch would need to do anyways), without any actual "switching".
Since all cases lead to the same label anyways, there is no need to switch once we know the value is in range of the switch.

Fixes:#14878

Disable the vs-integration tests from running with PRs by default.

Fix a crash caused by an improper reuse of lambdas from return inference cache.

Ensure that CPS property setters also invoke the code within ExecuteF…

Similar change as to TestUtilities.  Insteading of having two
independent projects with shared sources there are now a portable and
desktop only version.  The desktop version references the portable one.

Clean up the remaining solutions to have the utilities included.

Fix the factored out runtime code to properly execute the compiler
compiled test code in AppDomains.

Code move before refactoring it.

The Compilers.sln solution is compiling again and tests which don't
require AppDomain support are running again.  There is a bit of work
that is needed for AppDomains that I want to separate out.  In
particular the separation to portable means we have to redo some of the
serialization code.

The TestUtilities project should be our portable unit test helper.  This is part one of rearranging our sources to reflect that reality.  I'm separating it out because it involves a significant code move that
will make the actual rework part harder to review.

This is the project which will hold the CoreClr capabilities for our test infrastructure.  In particular it will hold the code for using AssemblyLoadContext to manage our isolated test boundaries.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=296550.

Updating the open vs-integration tests to run by default.

Add test for specified behavior of a virtual method invocation (via a binary compatibility scenario). (#16055)

* Add test for specified behavior of a virtual method invocation (via a binary compatibility scenario).