fstests: stop using /tmp directly

fstests: stop using /tmp directly

Tests should be using $tmp, not /tmp. this causes problems when
multiple tests all use /tmp/foo as a temporary test state file
and then step on each other.

Note that there are some tests that use /tmp to store "test stop"
files for background processes. Those that have proven to be
unreliable at stopping tests when interrupted by ctrl-c are also
updated to track and kill background processes in the cleanup
function.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>