Bug 1414399 - [mozbuild/moztest] Move mozbuild.testing.TestResolver to moztest.resolve r=gps

The TestMetadata and TestResolver classes aren't technically part of the build
system. The only connection is that they consume some build system output.

The next patch in this series is going to be merging in a bunch of other test
resolving logic from other parts of the tree. Moving this out first allows us
to keep that extra logic out of mozbuild.

MozReview-Commit-ID: 1eq4SjFVCyW

python/mach_commands.py

@@ -112,7 +112,7 @@ class MachCommands(MachCommandBase):
         # which produces output in the format Mozilla infrastructure expects.
         # Some tests are run via pytest.
         if test_objects is None:
-            from mozbuild.testing import TestResolver
+            from moztest.resolve import TestResolver
             resolver = self._spawn(TestResolver)
             if tests:
                 # If we were given test paths, try to find tests matching them.