tests: mark test-check-interfaces.py as requiring a repo stable

tests: mark test-check-interfaces.py as requiring a repo

This was failing our 4.6rc1 build like this:

  mercurial.error.RepoError: repository /tmp/build-debs.zMTRhC/src-4.6rc1 not found

Differential Revision: https://phab.mercurial-scm.org/D3425

--- a/tests/test-check-interfaces.py	Mon Apr 23 19:23:18 2018 +0100
+++ b/tests/test-check-interfaces.py	Tue Apr 24 08:20:15 2018 -0700
@@ -6,6 +6,13 @@
 encoding.environ[b'HGREALINTERFACES'] = b'1'
 
 import os
+import subprocess
+import sys
+
+# Only run if tests are run in a repo
+if subprocess.call(['python', '%s/hghave' % os.environ['TESTDIR'],
+                    'test-repo']):
+    sys.exit(80)
 
 from mercurial.thirdparty.zope import (
     interface as zi,