Reorganize unit-tests and test workflow

+ 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.

init.test.el

@@ -0,0 +1,7 @@
+;;; init.test.el -- for automated unit tests -*- lexical-binding: t; -*-
+
+(require 'core (concat user-emacs-directory "core/core"))
+
+(doom! :feature evil
+       :tools password-store
+       :private hlissner)