Add a test-sources configuration option. #4496

Job	Run time
Linters (mypy, flake8, etc.)	56s
Get qemu emulated architectures	9s
Test cibuildwheel building pyodide wheels	5m 56s
Test on ubuntu-latest (3.13)	9m 14s
Test on windows-latest (3.13)	8m 59s
Test on macos-13 (3.13)	9m 14s
Test on macos-14 (3.13)	9m 18s
Test on ubuntu-latest (3.8)	8m 58s
Test Linux aarch64 using qemu	26m 1s
Test Linux armv7l using qemu	9m 4s
Test Linux ppc64le using qemu	17m 26s
Test Linux s390x using qemu	18m 51s
	2h 4m 6s

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