selftests/exec: add a test for execveat()'s comm

In the previous patch we've updated AT_EMPTY_PATH execs to use the dentry
filename. Test for this and just to be sure keeps working with symlinks,
which was a concern in [1], I've added a test for that as well.

The test itself is a bit ugly, because the existing check_execveat_fail()
helpers use a hardcoded envp and argv, and we want to "pass" things via the
environment to test various argument values, but it seemed cleaner than
passing one in everywhere in all the existing tests.

Output looks like:

    ok 51 Check success of execveat(6, 'home/tycho/packages/...yyyyyyyyyyyyyyyyyyyy', 0)...
    # Check execveat(AT_EMPTY_PATH)'s comm is execveat
    ok 52 Check success of execveat(9, '', 4096)...
    # Check execveat(AT_EMPTY_PATH)'s comm is execveat
    ok 53 Check success of execveat(11, '', 4096)...
    # Check execveat(AT_EMPTY_PATH)'s comm is execveat
    [   25.579272] process 'execveat' launched '/dev/fd/9' with NULL argv: empty string added
    ok 54 Check success of execveat(9, '', 4096)...

Link: https://lore.kernel.org/all/20240925.152228-private.conflict.frozen.trios-TdUGhuI5Sb4v@cyphar.com/

 [1]
Signed-off-by: Tycho Andersen <tandersen@netflix.com>
Link: https://lore.kernel.org/r/20241030203732.248767-2-tycho@tycho.pizza


Signed-off-by: Kees Cook <kees@kernel.org>