refactor: use ReTest

[xukai92]

This PR replaces Test with ReTest (https://juliatesting.github.io/ReTest.jl/stable/) such that we can easily run a subset of tests.

Example

julia --project=@. -e 'using Pkg; Pkg.test(; test_args=ARGS)' "UnitEuclideanMetric/Leapfrog/Trajectory{MultinomialTS,FixedNSteps}/NoAdaptation"

Other benefits of ReTest

• It provides a better test summary with indication on which testsets miss a test.
• It provides a possible way to get rid of the hack to run AHMC tests vs Turing tests, but I will leave it for another PR.

Other changes include

• Removed Distributed (and related uses of module)---we actually never run the tests in a distributed manner and ReTest also supports a much simple way to handle this.
• Changed the default AHMC_TEST_GROUP to AdvancedHMC
• Added a simple CMD interface such that one can do julia --projects=@. tests/runtest.jl "TestName" to run dedicated tests.
• Disable CI for julia-nightly on Windows

[xukai92]

@yebai This is how the summary looks for test cases with no test: Note the yellow/orange zeros.

[yebai]

Hi, @xukai92 it looks good!