LU-15789 tests: execute cleanup_115() 1 time

LU-15789 tests: execute cleanup_115() 1 time

Patch fixes the following test_115() defects:
cleanup_115 () is called 3 times:
  1. at the end of test
  2. on run_one() returns
  3. on run_one() subshell exits

No reason to use $SINGLEMDS since test operates with mds1
only.
No reason to save FLAKEY since test is executed in subshell.
Correctly call DEBUGFS -c -R 'stat ROOT/$tdir/$tfile',
without correct quotation we have:
    debugfs 1.45.6.cr1 (14-Aug-2020)
    debugfs: No such file or directory while trying to open
      ROOT/d115.conf-sanity/f115.conf-sanity
      stat: Filesystem not open

Fixes: ef13d5464e ("LU-11915 tests: fix conf-sanity 115 test")
Fixes: d7449233c1 ("LU-8444 tests: test for unsigned xattr inode number")
Test-Parameters: trivial testlist=conf-sanity env=ONLY="115 116"
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-10901
Reviewed-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Change-Id: I11c4889ebe309c48527235c59fce1381202401b4
Reviewed-on: https://review.whamcloud.com/47153
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <artem.blagodarenko@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>