Automatically merge test profiles

[jimbru]

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

[elzibubble]

Merged this to my fork and published to https://clojars.org/org.clojars.lxsli/lein-test-out but it'd be great to get this in the official.