run-tests: test result shows when a failed test could not start a server

Failing to start a server happens regularly, at least on windows buildbot.
Such a failure often has nothing to do with the test, but with the environment.
But half the test output can change because some data is missing. Therefore this
is worth an extended error message.

Detect the server failure in the diff output because it is most reliable
there. Checking the output only does not show if the server failure was
expected.

 Old failure message when server start failed:
Failed test-serve.t: output changed

 New message:
Failed test-serve.t: serve failed and output changed

tests/run-tests.py

@@ -304,8 +304,14 b' def parsehghaveoutput(lines):'
 
 def showdiff(expected, output, ref, err):
     print
+    servefail = False
     for line in difflib.unified_diff(expected, output, ref, err):
         sys.stdout.write(line)
+        if not servefail and line.startswith(
+                             '+  abort: child process failed to start'):
+            servefail = True
+    return {'servefail': servefail}
+
 
 verbose = False
 def vlog(*msg):
@@ -1028,17 +1034,21 b' def runone(options, test, count):'
     elif ret == 'timeout':
         result = fail("timed out", ret)
     elif out != refout:
+        info = {}
         if not options.nodiff:
             iolock.acquire()
             if options.view:
                 os.system("%s %s %s" % (options.view, ref, err))
             else:
-                showdiff(refout, out, ref, err)
+                info = showdiff(refout, out, ref, err)
             iolock.release()
+        msg = ""
+        if info.get('servefail'): msg += "serve failed and "
         if ret:
-            result = fail("output changed and " + describe(ret), ret)
+            msg += "output changed and " + describe(ret)
         else:
-            result = fail("output changed", ret)
+            msg += "output changed"
+        result = fail(msg, ret)
     elif ret:
         result = fail(describe(ret), ret)
     else: