tests: clean up $DEBUGFS_EXE usage in scripts

tests: clean up $DEBUGFS_EXE usage in scripts

Instead of putting the entire test script under an implicit
"if test -x $DEBUGFS_EXE" conditional (sometimes indenting
the code, and sometimes not), rather check for the reverse
and exit the test script early if $DEBUGFS_EXE is missing.

In some places, tests were running $DEBUGFS_EXE directly,
when they should be running $DEBUGFS (which will run with
Fortify, or other options).

[ Fixed up missing exit statement in f_detect_junk. --tytso ]

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