tests: show skip reason instead of "irrelevant" with unified tests, too

parsehghaveoutput expects just the test output, not the merged test/output,
so for skipped unified tests e.g.:
Skipped test-convert-darcs.t: missing feature: irrelevant
was shown instead of:
Skipped test-convert-darcs.t: missing feature: darcs client

tests/run-tests.py

@@ -499,6 +499,9 b' def tsttest(test, options):'
         cmd = '/bin/sh "%s"' % name
         vlog("# Running", cmd)
         exitcode, output = run(cmd, options)
+        # do not merge output if skipped, return hghave message instead
+        if exitcode == SKIPPED_STATUS:
+            return exitcode, output
     finally:
         os.remove(name)