Bug 1744713: Manually add "xpcshell" to the sys.path for perftests r=sparky

Though _most_ of the old paths that used to be defined in
`mozperftest/runner.py` exist in `common_virtualenv_packages.txt`,
"xpcshell" was not because it's put in a different location depending on
whether the source directory is sourced from the "target.xpcshell.tests"
artifact or from VCS directly.

As part of this change, I've verified that all of the other
path changes in D132503 are fulfilled by the "mach" site.

Differential Revision: https://phabricator.services.mozilla.com/D133215

python/mozperftest/mozperftest/runner.py

@@ -77,6 +77,11 @@ def _activate_mach_virtualenv():
     )
     mach_site.activate()
 
+    if TASKCLUSTER:
+        # In CI, the directory structure is different: xpcshell code is in
+        # "$topsrcdir/xpcshell/" rather than "$topsrcdir/testing/xpcshell".
+        sys.path.append("xpcshell")
+
 
 def run_tests(mach_cmd, kwargs, client_args):
     """This tests runner can be used directly via main or via Mach.