tests: Avoid downloading deps in unit tests (#525)
tests: Avoid downloading deps in unit tests (#525)

- Uses custom `unit_tests` function in presubmit-tests.sh to run unit tests
 - Custom function adds a flag `-mod=vendor` in addition to default unit tests runner
 - The deps are already downloaded part of the source repo, mentioned flag uses it (vendor dir)
   and avoids downloading the deps afresh
 - This prevents(in unit tests) the flake we see 'go: error loading module requirements' which occurs
   due to download error
knative/client,knative/client