Add a test-sources configuration option. #4543

Job	Run time
Linters (mypy, flake8, etc.)	56s
Linters (mypy, flake8, etc.)	56s
Test on ubuntu-latest (3.13)	12m 3s
Test on ubuntu-latest (3.13)	54m 1s
Get qemu emulated architectures	10s
Get qemu emulated architectures	10s
Test on windows-latest (3.13)	12m 5s
Test on windows-latest (3.13)	52m 11s
Test cibuildwheel building pyodide wheels	4m 15s
Test cibuildwheel building pyodide wheels	3m 51s
Test on macos-13 (3.13)	12m 20s
Test on macos-13 (3.13)	34m 33s
Test on macos-14 (3.13)	12m 25s
Test on macos-14 (3.13)	17m 27s
Test on ubuntu-latest (3.8)	12m 22s
Test on ubuntu-latest (3.8)	52m 41s
Test Linux aarch64 using qemu	26m 42s
Test Linux aarch64 using qemu	26m 42s
Test Linux armv7l using qemu	9m 7s
Test Linux armv7l using qemu	9m 7s
Test Linux ppc64le using qemu	18m 28s
Test Linux ppc64le using qemu	18m 28s
Test Linux s390x using qemu	19m 11s
Test Linux s390x using qemu	19m 11s
	7h 9m 22s

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