Add unit tests

[Deraen]

Unit tests could help especially with the indent fn. There are probably a few cases broken currently: guns/vim-clojure-static#83

Not sure how these are usually done with Vimscript.

We could probably copy test cases from some other project for a start:
https://github.com/clojure-emacs/clojure-mode/blob/master/test/clojure-mode-indentation-test.el
https://github.com/weavejester/cljfmt/blob/master/cljfmt/test/cljfmt/core_test.cljc#L11

[Deraen]

The clojure code here already implements unit tests:

https://github.com/clojure-vim/clojure.vim/blob/master/clj/src/vim_clojure_static/test.clj
https://github.com/clojure-vim/clojure.vim/blob/master/clj/test/vim_clojure_static/indent_test.clj

We should setup CI to run these.

[Deraen]

Github actions configured in #4