Use XUnit for parallel testing.

[maleadt]

No description provided.

[maleadt]

Interestingly, this doesn't improve testing times at all:

Test
julia --project -e 'using Pkg; Pkg.test()'  221,61s user 1,62s system 100% cpu 3:42,95 total

XUnit.jl: single runner/thread
julia --project -e 'using Pkg; Pkg.test()'  239,89s user 2,20s system 99% cpu 4:03,54 total

XUnit.jl: 4 threads using ParallelTestRunner
julia --project -e 'using Pkg; Pkg.test()'  777,95s user 2,98s system 355% cpu 3:39,40 total

Presumably because this test suite is compilation heavy, and the Julia compiler is single-threaded.