Convert unit tests from busted/luarocks to use 'mineunit'.

[dennisjenkins75]

Per SX on Discord:

https://discord.com/channels/513329453741637637/809890578753650770/825866231835000892
https://discord.com/channels/513329453741637637/809890578753650770/826083288766218281

also still have not released "production" version so install command requires dev branch
luarocks install --server=https://luarocks.org/dev --local mineunit

qos has some clean tests with mineunit, might be good to look at:

• testing without dependencies to minetest: https://github.com/S-S-X/qos/blob/master/spec/queue_spec.lua
• very basic smoke tests, initialize mod and see if it crashes: https://github.com/S-S-X/qos/blob/master/spec/init_spec.lua
• testing with globalstep and minetest http api: https://github.com/S-S-X/qos/blob/master/spec/main_spec.lua
  Also if you want to integrate test runner to github pushes / pullrequests you can copy/paste to .github/workflows/mineunit.yml from comments: Onboarding / Howto / Readme S-S-X/mineunit#4 (comment)
  state of documentations for mineunit is not exactly good yet but enough stuff to actually run technic networks with it

Tried this and yes some changes would be needed.
tests -> spec is recommended (and maybe even required currently?) because mineunit uses only spec directory for tests and configurations by default.

Changes to test code are not that big, after renaming tests to spec:

After that luacheck however will complain about mineunit and sourcefile, no integration (yet) and so far I've simply added exclude_files = {"**/spec/**"} to .luacheckrc to not complain about test code at all.
changed busted . to mineunit --coverage --quiet && mineunit --report