Split different tests into submodules

[lgoettgens]

Still work in progress.

I try to wrap every logical block into its own module, as it is already the case for Piracy. This should help to be less confused about where some weird functions belong. And it declutters the tab-completion for Aqua..

[codecov]

Codecov Report

Merging #154 (7376d10) into master (94a9c7a) will increase coverage by 0.12%.
The diff coverage is 89.65%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   77.07%   77.19%   +0.12%     
==========================================
  Files          11       11              
  Lines         724      728       +4     
==========================================
+ Hits          558      562       +4     
  Misses        166      166              

Flag	Coverage Δ
unittests	77.19% <89.65%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Aqua.jl	73.07% <ø> (ø)
src/project_extras.jl	93.18% <50.00%> (-4.74%)	⬇️
src/piracy.jl	91.04% <55.55%> (ø)
src/undefined_exports.jl	90.90% <90.90%> (ø)
src/ambiguities.jl	90.82% <91.66%> (+1.22%)	⬆️
src/utils.jl	95.23% <96.66%> (+0.32%)	⬆️
src/deps_compat.jl	97.05% <100.00%> (+0.28%)	⬆️
src/project_toml_formatting.jl	87.50% <100.00%> (+1.78%)	⬆️
src/stale_deps.jl	97.56% <100.00%> (ø)
src/unbound_args.jl	80.00% <100.00%> (ø)

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