Bug 1269501: Part 1 - Add $PYTHON variable to environment for subprocess xpcshell tests. r=glandium

I'm not especially happy with this method, but the DMD tests are the only
other tests I can find doing anything like this, and I don't have a better
solution than doing it the same way.

MozReview-Commit-ID: JlclyKoUhRf

--HG--
extra : rebase_source : 61122aa9bb83a2d290a0e3daad34c78f96b82b7f
extra : source : ed35c5f4d756aa5b073832e21c103ce4faf282bd

testing/xpcshell/runxpcshelltests.py

@@ -650,6 +650,9 @@ class XPCShellTestThread(Thread):
             self.env['DMD_PRELOAD_VAR'] = preloadEnvVar
             self.env['DMD_PRELOAD_VALUE'] = libdmd
 
+        if self.test_object.get('subprocess') == 'true':
+            self.env['PYTHON'] = sys.executable
+
         testTimeoutInterval = self.harness_timeout
         # Allow a test to request a multiple of the timeout if it is expected to take long
         if 'requesttimeoutfactor' in self.test_object: