Bug 1958107: get rid of dependency on buildconfig in compare-mozconfigs test r=releng-reviewers,jcristau

I presume these used to run with a full checkout + configure; this is no longer true.

Differential Revision: https://phabricator.services.mozilla.com/D244653

build/compare-mozconfig/compare-mozconfigs.py

@@ -10,9 +10,10 @@ import logging
 import os
 import unittest
 
-import buildconfig
 import mozunit
 
+here = os.path.abspath(os.path.dirname(__file__))
+
 FAILURE_CODE = 1
 SUCCESS_CODE = 0
 
@@ -168,7 +169,7 @@ def compare(topsrcdir):
 
 class TestCompareMozconfigs(unittest.TestCase):
     def test_compare_mozconfigs(self):
-        topsrcdir = buildconfig.substs["top_srcdir"]
+        topsrcdir = os.path.abspath(os.path.join(here, "..", ".."))
         self.assertTrue(compare(topsrcdir))