Organize tests

[hyrodium]

The current test/runtests.jl has 883 lines, and it is a bit hard to maintain. This PR organizes the test.

• Better visibility with better indents.
• Split runtests.jl.
• Add missing tests. e.g. missing @test in https://github.com/JuliaMath/IntervalSets.jl/blob/2779dd11bda0c8ceecac25195dc69b3f73074358/test/runtests.jl#L832-L841
• Use @iv_str macro for better visibility.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (13c76e1) 99.11% compared to head (1b4cacd) 98.84%. Report is 9 commits behind head on master.

Files	Patch %	Lines
ext/IntervalSetsRecipesBaseExt.jl	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #184 +/- ## ========================================== - Coverage 99.11% 98.84% -0.27% ========================================== Files 6 6 Lines 225 260 +35 ========================================== + Hits 223 257 +34 - Misses 2 3 +1 ```

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