run-tests: record skips by raising SkipTest

The unittest way of recording a skipped test is to raise an exception
(at least with modern unittest implementation). We change Test to raise
a SkipTest when operating in unittest mode.

This does prevent some "tear down" activities from running in unittest
mode. This will be fixed in subsequent patches. Since unittest mode
is experimental, this should be OK.

tests/run-tests.py

@@ -607,6 +607,9 b' class Test(object):'
         return warned and '~' or '!', self.name, msg
 
     def skip(self, msg):
+        if self._unittest:
+            raise SkipTest(msg)
+
         if self._options.verbose:
             log("\nSkipping %s: %s" % (self._path, msg))
 
@@ -979,6 +982,9 b' def run(cmd, wd, options, replacements, '
 
 iolock = threading.Lock()
 
+class SkipTest(Exception):
+    """Raised to indicate that a test is to be skipped."""
+
 class TestResult(unittest._TextTestResult):
     """Holds results when executing via unittest."""
     # Don't worry too much about accessing the non-public _TextTestResult.
@@ -1334,6 +1340,8 b' class TestRunner(object):'
                     self.runTest()
                 except KeyboardInterrupt:
                     raise
+                except SkipTest, e:
+                    result.addSkip(self, str(e))
                 except self.failureException:
                     result.addFailure(self, sys.exc_info())
                 except Exception:
@@ -1348,13 +1356,11 b' class TestRunner(object):'
                     self._result.failures.append((self, msg))
                 elif code == '~':
                     self._result.addWarn(self, msg)
-                elif code == '.':
-                    # Success is handled automatically by the built-in run().
-                    pass
-                elif code == 's':
-                    self._result.addSkip(self, msg)
                 elif code == 'i':
                     self._result.addIgnore(self, msg)
+                # Codes handled in run().
+                elif code in ('.', 's'):
+                    pass
                 else:
                     self.fail('Unknown test result code: %s' % code)