test-systemd-tmpfiles: ignore result of %m test

test-systemd-tmpfiles: ignore result of %m test

It's failing on artful s390x and i386:
Running /tmp/autopkgtest.Pexzdu/build.lfO/debian/build-deb/systemd-tmpfiles  on 'f /tmp/test-systemd-tmpfiles.c236s1uq/arg - - - - %m'
expect: '01234567890123456789012345678901'
actual: 'e84bc78d162e472a8ac9759f5f1e4e0e'
--- stderr ---
Traceback (most recent call last):
  File "/tmp/autopkgtest.Pexzdu/build.lfO/debian/src/test/test-systemd-tmpfiles.py", line 129, in <module>
    test_valid_specifiers(user=False)
  File "/tmp/autopkgtest.Pexzdu/build.lfO/debian/src/test/test-systemd-tmpfiles.py", line 89, in test_valid_specifiers
    test_content('f {} - - - - %m', '{}'.format(id128.get_machine().hex), user=user)
  File "/tmp/autopkgtest.Pexzdu/build.lfO/debian/src/test/test-systemd-tmpfiles.py", line 84, in test_content
    assert content == expected
AssertionError
-------

Let's skip the test for now until this is resolved properly on the autopkgtest
side.