tests: Handle backup command exceptions as test failures, not errors

tests: Handle backup command exceptions as test failures, not errors

If the backup command fails (i.e. throws an exception), we want the test
to fail. This makes it easier to mark tests as 'knownfail' (because we
can't knownfail test errors).

In theory, this should just involve updating run_cmd() to catch any
exceptions from the command and then call self.fail().

However, if the backup command fails, it can leave behind files in the
targetdir. Partly this is intentional, as these files may provide clues
to users as to why the command failed. However, in selftest, it causes
the TestCaseInTempDir._remove_tempdir() assertion to fire. Because this
assert actually gets run as part of the teardown, the assertion gets
treated as an error rather than a failure (and so we can't knownfail the
backup tests). To get around this, we remove any files in the tempdir
prior to calling self.fail().

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>