test_compare: add config_tmpdir fixture to block ignore-branches

Isolate test_compare() from the environment's west config. This makes
the test pass again when the user sets `west config --global
ignore-branches True`. It was failing like this:

```
  # By default, a checked-out branch should print output, even if
  # the tree is otherwise clean...
  check_output(['git', 'checkout', '-b', 'mybranch'], cwd=kconfiglib)
  actual = cmd('compare')
>    assert actual.startswith('=== Kconfiglib (subdir/Kconfiglib):')
E    AssertionError: assert False
```

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

tests/test_project.py

@@ -259,7 +259,7 @@ def test_manifest_freeze(west_update_tmpdir):
         assert re.match(eline_re, aline) is not None, (aline, eline_re)
 
 
-def test_compare(west_init_tmpdir):
+def test_compare(config_tmpdir, west_init_tmpdir):
     # 'west compare' with no projects cloned should still work,
     # and not print anything.
     assert cmd('compare') == ''