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 without having to manually using 'with-profile'.

[*]
https://github.com/technomancy/leiningen/blob/90bf2d47cd0ef113ab6d1a498608a917d586ad6c/src/leiningen/test.clj#L192