pkgdev tatt: forces USE=test

`pkgdev tatt` will try to set `USE=test` for a package which confuses things. There shouldn't be a need for it. With `emerge --with-test-deps foo`, test dependencies will be pulled in (so no need for `USE=test` for that). For `FEATURES=test emerge foo`, `USE=test` is automatically enabled _if tests are not restricted_). Forcing `USE=test` breaks where tests are restricted but building tests fails for some other reason which has gone unnoticed because of that restriction, and it is unlikely that any developers will be interested in this. To silence reports about this, developers would have to p.use.mask the `test` USE flag for a package _as well_, but this also confuses `emerge --with-test-deps`! One example recently is https://bugs.gentoo.org/967274. TL;DR: We shouldn't ever set `USE=test` in `pkgdev tatt`.