wrapper3: add unittests

This module has some complicated logic which begs for unittests.

BUG=chromium:1170007
TEST=CQ passes

Change-Id: I080aa2599a049f533f70258c7ba4f21099489afa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2692518
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/scripts/wrapper3.py b/scripts/wrapper3.py
index 7ee9785..f61f9a2 100755
--- a/scripts/wrapper3.py
+++ b/scripts/wrapper3.py
@@ -61,6 +61,7 @@
     path = os.path.dirname(os.path.realpath(__file__))
     while not os.path.exists(os.path.join(path, 'PRESUBMIT.cfg')):
       path = os.path.dirname(path)
+      assert path != '/', 'Unable to locate chromite dir'
 
     # pylint: disable=global-statement
     global CHROMITE_PATH