Split tests into four groups for CI job matrices

[ali-ramadhan]

This PR splits the tests into four test groups, selectable with the TEST_GROUP environment variable: unit, integration, regression, and scripts. Using TEST_GROUP=all (the default if TEST_GROUP is not defined) will run all tests.

The purpose of this PR is to address long test build times (see #860) that time out at 50 minutes on Travis CI and at 60 minutes on GitLab CI by splitting tests into multiple jobs (i.e. building a job matrix) that each should individually run much faster.

If the different jobs could be run in parallel this would speed up our test builds significantly, but alas we are stuck with free-tier CI pipelines so we can't run too many jobs in parallel and will probably end up waiting longer. But at least our jobs won't time out.

In working on this PR I was also able to revive the Appveyor and Docker CI pipelines (I think Appveyor will only show up for future PRs).

Resolves #139

[codecov]

Codecov Report

Merging #872 into master will decrease coverage by 1.40%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
- Coverage   72.50%   71.09%   -1.41%     
==========================================
  Files         187      187              
  Lines        5331     5217     -114     
==========================================
- Hits         3865     3709     -156     
- Misses       1466     1508      +42     

Impacted Files	Coverage Δ
src/AbstractOperations/AbstractOperations.jl	33.33% <0.00%> (-33.34%)	⬇️
src/AbstractOperations/interpolation_utils.jl	63.33% <0.00%> (-30.01%)	⬇️
src/Oceananigans.jl	75.00% <0.00%> (-25.00%)	⬇️
src/Solvers/batched_tridiagonal_solver.jl	60.71% <0.00%> (-24.29%)	⬇️
src/Forcing/simple_forcing.jl	81.81% <0.00%> (-18.19%)	⬇️
test/runtests_utils.jl	50.00% <0.00%> (-17.57%)	⬇️
src/Utils/versioninfo.jl	68.75% <0.00%> (-16.97%)	⬇️
...closure_implementations/anisotropic_diffusivity.jl	84.00% <0.00%> (-16.00%)	⬇️
src/Buoyancy/Buoyancy.jl	63.15% <0.00%> (-15.79%)	⬇️
src/TurbulenceClosures/closure_operators.jl	86.20% <0.00%> (-13.80%)	⬇️
... and 60 more

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[ali-ramadhan]

Alright I think our CI is back online: Travis, GitLab, Appveyor, and Docker CI all work again. And I think CodeCov should be more lenient about failing PRs now so we won't get red crosses on most PRs anymore.

Tests will take longer to run now though.

[ali-ramadhan]

Found a trick to configure Travis CI to build pull requests but only pushes to master (thus reducing redundancy as default Travis CI behavior is to build both pushes and merge commits on PRs).

See: https://stackoverflow.com/a/31882307

[glwagner]

Successful in 65m...