LU-1538 tests: clean up runtests code style

LU-1538 tests: clean up runtests code style

Use $(...) instead of `...` for sub-shells.
Improve some error messages and code style.
Use $DIR/$tdir for tests instead of $MOUNT/$$.

Test-Parameters: envdefinitions=SLOW=yes  testlist=runtests
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I4f3eb4a3e288c6fc45ae6ef1fb2008e8e6babe84
Reviewed-on: http://review.whamcloud.com/5739
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>