tests/shell: print "kernel is tainted" separate from test result

tests/shell: print "kernel is tainted" separate from test result

Once the kernel is tainted, it stays until reboot. It would not be
useful to fail the entire test run based on that (and we don't do that).

But then, it seems odd to print this in the same style as the test
results, because a [FAILED] of a test counts as an overall failure.
Instead, print this warning in a different style.

Previously:

    $ ./tests/shell/run-tests.sh -- /usr/bin/true
    ...

    W: [FAILED]     kernel is tainted
    I: [OK]         /usr/bin/true

    I: results: [OK] 1 [SKIPPED] 0 [FAILED] 0 [TOTAL] 1

Now:

    $ ./tests/shell/run-tests.sh -- /usr/bin/true
    ...

    W: kernel is tainted

    I: [OK]         /usr/bin/true

    I: results: [OK] 1 [SKIPPED] 0 [FAILED] 0 [TOTAL] 1

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>