flag to enable stylesheets in karma tests

from https://gitlab.com/gitlab-org/gitlab-ce/issues/49777#note_91737030

To help debug karma specs, useful to see them with CSS applied

running ENABLE_STYLESHEETS=1 karma:fixtures should include code that was removed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7589/diffs?commit_id=d9fe5c259d8880fcb9f8c1cc3322836d4d35ef93