Changed Pkg.test to prioritise julia_args --threads over current process threads #4314

Job	Run time
docs	4m 12s
test (ubuntu-latest, x64, pkg.julialang.org, nightly)	18m 39s
test (ubuntu-latest, x86, pkg.julialang.org, nightly)	26m 17s
test (macOS-14, aarch64, pkg.julialang.org, nightly)	15m 18s
test (windows-latest, x64, pkg.julialang.org, nightly)	35m 41s
test (windows-latest, x86, pkg.julialang.org, nightly)	45m 50s
test (ubuntu-latest, x64, nightly)	21m 47s
	2h 47m 44s

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