In 99cdc069e2, we added a check for
whether the directory the user was attempting to test existed or not.
However, adding this check prevented us from running `make test
WHAT=./path/to/pkg/...`, because we believed that `./path/to/pkg/...`
wasn't a valid path to a directory.
Fix this by stripping `/...` before checking whether the directory
exists.
2d3ad78cf9