Adds an INSTALLED-ONLY-P argument to doom-get-packages.
Fixes a missing-package error when doom-get-outdated-packages iterates
over packages that aren't installed.
Plus, updated unit tests.
+ Moved unit tests out of tests/ and into their respective modules.
+ Rewrite makefile and added these tasks:
+ <MODULE>/<SUBMODULE> -- byte-compile a specific module
+ test:<MODULE>/<SUBMODULE> -- runs tests for a specific module
+ testi -- run tests in an interactive session of Emacs (WIP)
+ run -- opens an Emacs session with this config; useful when it is in
a non-standard location.
