MR !1141 broke the way stop.pl is invoked when start.pl fails:

  - start.pl changes the working directory to $testdir/$server before
    attempting to start $server,

  - commit 27ee629e causes the $testdir
    variable in stop.pl to be determined using the $SYSTEMTESTTOP
    environment variable, which is set to ".." by all tests.sh scripts,

  - commit e227815a makes start.pl pass
    $test (the test's name) rather than $testdir (the path to the test's
    directory) to stop.pl when a given server fails to start.

Thus, when a server is restarted from within a tests.sh script and such
a restart fails, stop.pl attempts to look for the server directory in a
nonexistent location ($testdir/$server/../$test, i.e. $testdir/$test,
instead of $testdir/../$test).  Fix the issue by changing the working
directory before stop.pl is invoked in the scenario described above.