Move compiler tests to Compiler package

[Keno]

This does not yet make the compiler tests independently runnable using ] test Compiler; it only moves the files and wires them up to continue running as part of the Base test runner.

[KristofferC]

To make them runnable from Pkg we only need to add a runtests.jl file as well as a https://github.com/JuliaLang/julia/blob/master/stdlib/LinearAlgebra/test/testgroups file so that when Base CI run this are run in parallel? It is a bit unfortunate that test Compiler will run them serially but there is no built in support for running package tests in parallel.

[Keno]

Yes, this PR does add those files. The parallel running is an issue, but the bigger issue right now is that a test Compiler will test Base.Compiler, not the package version. This needs a larger adjustment to each test, which will be in a follow up PR.