We already refer to test targets as "test targets" instead of "tests" in
`--test` but not `--tests` or in the error output. This makes it
uniformly refer to them as "test targets", making it clearer that these
aren't test functions.
Fixes#7864
This mirrors some of the categories from `cargo help` (the man pages).
There are fewer categories to avoid extra vertical space. Instead, they
are left int the `Options` category but put first.