Changed Pkg.test to prioritise julia_args --threads over current process threads #4324

Job	Run time
docs	4m 17s
test (ubuntu-latest, x64, pkg.julialang.org, nightly)	20m 42s
test (ubuntu-latest, x86, pkg.julialang.org, nightly)	25m 39s
test (macOS-14, aarch64, pkg.julialang.org, nightly)	17m 22s
test (windows-latest, x64, pkg.julialang.org, nightly)	31m 58s
test (windows-latest, x86, pkg.julialang.org, nightly)	37m 8s
test (ubuntu-latest, x64, nightly)	23m 38s
	2h 40m 44s

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