Add a test-sources configuration option. #4497

Job	Run time
Linters (mypy, flake8, etc.)	56s
Get qemu emulated architectures	12s
Test cibuildwheel building pyodide wheels	5m 40s
Test on ubuntu-latest (3.13)	43m 38s
Test on windows-latest (3.13)	52m 46s
Test on macos-13 (3.13)	20m 12s
Test on macos-14 (3.13)	12m 41s
Test on ubuntu-latest (3.8)	44m 43s
Test Linux aarch64 using qemu	26m 9s
Test Linux armv7l using qemu	9m 20s
Test Linux ppc64le using qemu	17m 34s
Test Linux s390x using qemu	19m 27s
	4h 13m 18s

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