Bug 1445944 - [moztest] Update shared test fixtures so they can work outside of mozilla-central r=rwood

This isn't strictly related to this bug, but it is a change made to mozbase in
the raptor repo that is worth backporting here. Figured it's easiest to land it
alongside the other mozbase backports.

MozReview-Commit-ID: DW7I2zKZZNk

--HG--
extra : rebase_source : a000d27774f224d37f981d6683d96c65846b8a32

testing/mozbase/moztest/moztest/selftest/fixtures.py

@@ -15,10 +15,14 @@ import mozfile
 import mozinstall
 import pytest
 import requests
-from mozbuild.base import MozbuildObject
 
 here = os.path.abspath(os.path.dirname(__file__))
-build = MozbuildObject.from_environment(cwd=here)
+
+try:
+    from mozbuild.base import MozbuildObject
+    build = MozbuildObject.from_environment(cwd=here)
+except ImportError:
+    build = None
 
 
 HARNESS_ROOT_NOT_FOUND = """
@@ -29,9 +33,10 @@ environment variable is required.
 
 def _get_test_harness(suite, install_dir):
     # Check if there is a local build
-    harness_root = os.path.join(build.topobjdir, '_tests', install_dir)
-    if os.path.isdir(harness_root):
-        return harness_root
+    if build:
+        harness_root = os.path.join(build.topobjdir, '_tests', install_dir)
+        if os.path.isdir(harness_root):
+            return harness_root
 
     # Check if it was previously set up by another test
     harness_root = os.path.join(os.environ['PYTHON_TEST_TMP'], 'tests', suite)
@@ -113,3 +118,6 @@ def binary():
         bindir = mozinstall.install(
             os.environ['GECKO_INSTALLER_URL'], os.environ['PYTHON_TEST_TMP'])
         return mozinstall.get_binary(bindir, app_name='firefox')
+
+    if 'GECKO_BINARY_PATH' in os.environ:
+        return os.environ['GECKO_BINARY_PATH']