[tests] Test for GC descriptors - first draft.

This runs for a long time and potentially uses lots of memory.  It's not integrated
in the test suite and must be run manually.
[tests] Optional big-switch version of GC descriptor test.

This takes ages to compile with mcs.  See bug #10781.
[tests] Test valuetype GC descriptors, too.
Merge branch 'feature-descriptor-tests'