tests: kill debugfs on interrupted MMP test

tests: kill debugfs on interrupted MMP test

If the f_mmp test is interrupted during its test run, then it can
leave debugfs busy-looping in the background.  Since f_mmp is a
relatively long-running test, and is likely to be running during
a parallel test run, this can happen fairly often.

Set a signal trap for the f_mmp test script being killed, so that
the background debugfs command will always be killed by the test.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>