tests: kill process group on failure

Test suite is often leaving `stray` processes on failure. They are
harmless, but may cause future test runs fail because those `stray`
processes are occupying `common.PORT` or due to some other reasons.

Killing whole process group on test suite failure should help in such
cases.

tools/test.py

@@ -1437,4 +1437,10 @@ def Main():
 
 
 if __name__ == '__main__':
-  sys.exit(Main())
+  ret = 0
+  try:
+    ret = Main()
+    sys.exit(ret)
+  finally:
+    if ret and not utils.IsWindows():
+      os.killpg(0, signal.SIGKILL)