Enable selective test execution in Mill's own CI #2933

Job	Run time
test-docs	7m 5s
build-windows / run	2m 52s
build-linux / run	3m 45s
windows (11, "main.__.test") / run	6m 4s
lint-autofix / run	7m 22s
itest (11, ci/test-mill-dev.sh && ci/test-mill-release.sh && ./mill -i -k __.ivyDepsTree && ./mil... / run	5m 51s
itest (17, ci/test-mill-bootstrap.sh) / run	9m 43s
linux (17, 'main.__.test', false) / run	4m 43s
	47m 25s

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