Make $objdir/_tests/testing/mochitest/runtests.py runnable from any directory, not just $objdir/_tests/testing/mochitest/. r=sayrer, NPOTB and all that anyway

testing/mochitest/runtests.py.in

@@ -75,6 +75,9 @@ BROWSER_CHROME_URL = "chrome://browser/content/browser.xul"
 # (particularly after a build) takes forever.
 SERVER_STARTUP_TIMEOUT = 45
 
+SCRIPT_DIRECTORY = os.path.abspath(os.path.realpath(os.path.dirname(sys.argv[0])))
+os.chdir(SCRIPT_DIRECTORY)
+
 PROFILE_DIRECTORY = os.path.abspath("./mochitesttestingprofile")
 
 LEAK_REPORT_FILE = PROFILE_DIRECTORY + "/" + "leaks-report.log"