Fix test module imports (#277)

This commit modifies `top_level_dir`, which is used to resolve module names
when loading unittests, to ensure, only modules from specified ST package are
importet.

Without this commit, UnitTesting might attempt to import packages/modules/tests
from Lib/ directory due to possible ambiguities.

Scenario:

Plugins of `MyPackage` are organized in `Packages/MyPackage/dirname/` and tests
in `Packages/MyPackage/dirname/tests/`. If a library `Lib/pythonXX/dirname`
exists, Unittesting would have attempted to import tests from that directory,
because test module paths would have been translated to `dirname.tests` due to
`top_level_dir` pointing to `Packages/MyPackage`.

By adjusting `top_level_dir` to `Packages/`, test module names are translated
to `MyPackage.dirname.tests` instead.

Author: deathaxe

unittesting/unit.py

@@ -221,7 +221,7 @@ def run_tests(self, stream, package, settings, cleanup_hooks):
                 loader = DeferrableTestLoader(settings["deferred"])
                 if os.path.exists(os.path.join(start_dir, "__init__.py")):
                     tests = loader.discover(
-                        start_dir, settings["pattern"], top_level_dir=package_dir
+                        start_dir, settings["pattern"], top_level_dir=sublime.packages_path()
                     )
                 else:
                     tests = loader.discover(start_dir, settings["pattern"])