Convert tests to functional form

[odow]

Closes #110
Closes #108

Another quite large diff that is just moving things around. This converts all the tests into the function test_ form to remove global state (some test files were just one large global script!).

I also removed the doc examples from the test script, since these get run and tested when the docs are built:

DiffOpt.jl/docs/make.jl

Lines 33 to 37 in 190beb8

	# `include` the file to test it before `#src` lines are removed. It is
	# in a testset to isolate local variables between files.
	@testset "$(filename)" begin
	_include_sandbox(filename)
	end

and not all tutorials were tested anyway.

This let's us remove a bunch of packages from the tests like Flux and Plots, which should help speed things up.

I haven't addressed #195. That can wait for another PR.

Also, I noticed that there are a lot of comments in the tests with various things commented out and TODOs. I've left for now, but there are lot of open issues related to testing (#202, #109, #108, #107, #106, #102) , so the whole package could use some more work on that front.

[odow]

And here's a reason to refactor the tests.

Tests are failing because it looks like https://github.com/jump-dev/DiffOpt.jl/blob/master/test/sensitivity_index_issue.jl was never actually tested.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -1.05 ⚠️

Comparison is base (190beb8) 87.92% compared to head (2574dfb) 86.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   87.92%   86.88%   -1.05%     
==========================================
  Files          11       11              
  Lines        1052     1052              
==========================================
- Hits          925      914      -11     
- Misses        127      138      +11     

see 2 files with indirect coverage changes

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[matbesancon]

Thanks!

[blegat]

@odow Why removing the test/Project.toml here ? I find it quite useful for testing things locally compared to the extra section in the root Project.toml file

[odow]

I never use it, and it can cause confusing bugs: JuliaPolyhedra/CDDLib.jl#82.

But we can add it back...