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.

core/core-popups.el

@@ -73,6 +73,7 @@ is enabled/disabled.'")
           ("^\\*doom:" :regexp t :size 0.35 :noesc t :select t :modeline t)
           ("^\\*doom " :regexp t :noselect t :autokill t :autoclose t)
           ;; built-in (emacs)
+          ("*ert*" :same t :modeline t)
           ("*info*" :size 0.5 :select t :autokill t)
           ("*Backtrace*" :size 20 :noselect t)
           ("*Warnings*"  :size 8  :noselect t)