When we do a release type build of the server (with both optimized and
debug enabled server/plugins) with Developer Studio, some MTR tests
when run with --debug-server will fail in one of two ways:

1. Tests which try to load a plugin into the mysql client fail with
   missing symbols. This is caused by the plugin having references to
   functions which do not exist in the non-debug client.

2. Some tests on sparc fail with Thread stack overrun.

Fix for issue #1: mtr will have appended /debug to the plugin dir part
when running with --debug-server and if there actually is such a
directory. The fix is to remove any trailing /debug from the
env. variable within the test. This will affect the client only, not
the server. Developer builds will not have put the plugins in a
subdirectory /debug so it makes no different to those.

Fix for issue #2: apparently this thread stack overrun is not feasible
to avoid, so just skip the test if running with debug server on sparc;
there is already an include file to do that.

Also added not_sparc_debug.inc to the "white list" so the tests are
skipped even when running mtr --no-skip.

(cherry picked from commit 9c79e477261ab252e38def436bca3336ef597603)