run-tests: make coverage work out of tree

Currently coverage fails when run on an out-of-tree extension since
run-tests.py tries to load sitecustomize.py from self._testdir, which is the
dir for the extension's tests.

Differential Revision: https://phab.mercurial-scm.org/D6960

tests/run-tests.py

@@ -3170,7 +3170,7 b' class TestRunner(object):'
                 print('WARNING: cannot fix hg.bat reference to python.exe')
 
         if self.options.anycoverage:
-            custom = os.path.join(self._testdir, 'sitecustomize.py')
+            custom = os.path.join(osenvironb[b'RUNTESTDIR'], 'sitecustomize.py')
             target = os.path.join(self._pythondir, 'sitecustomize.py')
             vlog('# Installing coverage trigger to %s' % target)
             shutil.copyfile(custom, target)