Refactoring base's runtests harness for packages to use too

[tkelman]

Ref #17165 (comment), we should try to refactor what's currently in test/runtests.jl to instead be a default part of the Base.Test module so packages can also use it for pretty-printing, parallel execution, running of selective subsets of tests (related to #15404), and everything else that base's tests do but packages have to roll their own.

[kshyatt]

OK, so you ought to be able to give runtests function two args: a test dir (which may be base Julia test/ or joinpath(Pkg.dir(), test)) and a list of test files to run - and if we have that, I think it should work? We might want to include sharding (like we have now for base, where all linalg tests run first)? Anything else?