Reorganize tests and update CI

[devmotion]

This PR separates the tests of Turing + Soss a bit more and fixes #161. I moved them to a separate action such that CI tests are still shown as passing, also in the README, if the Turing + Soss tests fail 🙂

I also thought it's sufficient to run the PPL tests with the stable Julia version on Linux.

[codecov-commenter]

Codecov Report

Merging #162 (b6e4c33) into master (5bdd823) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   99.23%   99.23%   -0.01%     
==========================================
  Files          10        9       -1     
  Lines         262      260       -2     
==========================================
- Hits          260      258       -2     
  Misses          2        2              

Impacted Files	Coverage Δ
src/abstract_gp/finite_gp.jl	100.00% <0.00%> (ø)
src/AbstractGPs.jl

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[willtebbutt]

This is great. Please feel free to merge when CI passes.

[devmotion]

BTW tests with Julia nightlies are broken since isgenerated was renamed to hasgenerator on the master branch but IRTools uses isgenerated. It seems it would be fixed by FluxML/IRTools.jl#86 (or by reverting the change in base or at least re-adding isgenerated, there's some discussion in the original PR it seems).