tests: don't skip case "output undefined"

tests: don't skip case "output undefined"

Treat missing expected files as empty and let the test fail if
there is non-empty output.

Expected output may be missing in these cases:
 1. forgot to commit the file after changing/adding a (sub)test
 2. a bug in a test where we do tricks with TS_EXPECTED
 3. and most notable if ts_die() is called before a subtest is
    initialized, e.g. in ts_scsi_debug_init()

I always wondered why we don't treat this as FAILED. Now we do
so, ts_finalize and ts_gen_diff looks much cleaner now.

The change discovers that tests with subtest were ignoring the
"non-sub" expected files which had to be fixed. BTW we removed
any zero sized files.

Moreover now we respect diff's return value. In past all test
succeeded when diff was not able to write to diffdir, e.g. when
running tests as normal user after they run as root.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>