Fixes test failures introduced with new version of packaged argcomplete.
Fixes test failures introduced with new version of packaged argcomplete.

Note that this behavior only affected tests, which were lacking the
_ARGCOMPLETE_COMP_WORDBREAKS environment variable. Tab completion
functionality outside of tests remains unchanged.

GoogleCloudPlatform/gsutil,BrandonY/gsutil,GoogleCloudPlatform/gsutil,fishjord/gsutil