Don't leave `path` behind when executing `:chflags_success`

[ngie]

Summary

Prior to this change a SF_IMMUTABLE chflagsat(2)'ed file (path) was left
behind, which sabotaged kyua(1) from being able to clean up the work directory,
This resulted in unnecessary work for folks having to clean up the work
directory on non-disposable systems, which defaults to /tmp. Use UF_OFFLINE
instead of SF_IMMUTABLE, in part because setting SF_IMMUTABLE isn't relevant
to the test and SF_IMMUTABLE cannot be cleared at all securelevels, as pointed
out by @asomers.

Additional work is required to catch cases like this upfront in the future to
avoid tester headache. See https://github.com/jmmv/kyua/issues/142 for more details.

Suggested by: asomers
MFC after: 1 week
PR: 247761
Sponsored by: DellEMC

Test Plan

$ sudo sh -c 'chflags -R 0 /tmp/kyua.*; rm -Rf /tmp/kyua.*'
chflags: /tmp/kyua.*: No such file or directory
$ sudo kyua test -k /usr/tests/sys/audit/Kyuafile file-attribute-modify
...

Results file id is usr_tests_sys_audit.20200711-145256-203233
Results saved to /root/.kyua/store/results.usr_tests_sys_audit.20200711-145256-203233.db

53/53 passed (0 failed)
$ ls /tmp/kyua*
ls: /tmp/kyua*: No such file or directory
$

[asomers]

Instead of clearing flags, which might not be possible in a higher securelevel, how about just using UF_OFFLINE instead of SF_IMMUTABLE? That's what some of the other test cases do. The test doesn't care what the flag actually is; it just needs to be something.

[ngie]

In D25561#565211, @asomers wrote:

Instead of clearing flags, which might not be possible in a higher securelevel, how about just using UF_OFFLINE instead of SF_IMMUTABLE? That's what some of the other test cases do. The test doesn't care what the flag actually is; it just needs to be something.

Ack. That reasoning makes a whole lot of sense :).

[ngie]

Use UF_OFFLINE instead of SF_IMMUTABLE, as recommended by @asomers.

[ngie]

tests/sys/audit/file-attribute-modify.c

714

I'll remove this spurious whitespace change in the commit.