Add check for left-over testsuite/.git folder to sync-all

This way, the first time sync-all is called after updating to a
post-testsuite-merge (see #8545) state of ghc.git, the sync-all script
aborts with an error message if a testsuite/.git folder is detected and
thus forces the user to take action.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>