Automatically merge test profiles

When Leiningen runs its built-in 'test' command, it merges both the
`:test` and `:leiningen/test` profiles automatically. If a project has
test-only configuration enabled under either of those profiles, simply
running `$ lein test-out ...` will fail. Rather, the user must manually
enable the needed profiles by running under 'with-profile'. This commit
automatically merges the same profiles that `lein test` does to maintain
compatibility.