Refactor the testsuite to be more fine grained in assignment of tests to workers

[ViralBShah]

The test/sparse.jl is a huge file. Splitting it into a few smaller files will possibly help alleviate JuliaLang/julia#45754 and also leverage more processors during tests.

[rayegun]

Does each file need its own module?

[ViralBShah]

I don't think so. I checked that the existing sparse tests do run across multiple workers. Seems like the julia runtests will pick each file to run in its own worker. The modules probably do help to avoid accidental reuse of global variables and that sort of stuff.

[ViralBShah]

How about just get rid of the testgroups stuff since the solvers have to be manually included anyways? Also maybe move the multi-threaded execution test into solvers/threads.jl?

[DilumAluthge]

You need the testgroups file, otherwise Base won't split your test suite up into multiple groups.

[ViralBShah]

I don't see testgroups files in most of the tests though, which made me wonder. I thought it would be a lot more common.

➜  julia git:(master) ✗ find . -name testgroups
./stdlib/Dates/test/testgroups
./stdlib/SparseArrays-2bbdd7a12ead8207593655c541ba347761a9c663/test/testgroups
./stdlib/LinearAlgebra/test/testgroups
./stdlib/LibGit2/test/testgroups

[DilumAluthge]

Yeah, most stdlibs don't have a testgroups file, but we probably should add them to more stdlibs.

We definitely want to have them in the linear algebra-related stdlibs, because those tests take so long.

[ViralBShah]

How to handle the USE_GPL_LIBS with testgroups?

[DilumAluthge]

Hmmm. Presumably, USE_GPL_LIBS is true by default in vanilla build of Julia?

[ViralBShah]

It is - but we do need to support the flag and not assume it's always present. I suppose it is ok if the sparse solver tests all run in one process.

[DilumAluthge]

I think you'll need to add an if Base.USE_GPL_LIBS guard to the top of each test file.

[ViralBShah]

Fixed by #151 and #154