fix: warning when building macOS arm64 wheels

[mayeut]

When MACOSX_DEPLOYMENT_TARGET is not set by the user, cibuildwheel always defaults to 10.9. This leads to a warning when building arm64 wheels.
This commit removes the warning by setting the default to 11.0 for arm64 builds.

Reported in #1300 (comment)

[mayeut]

I chose to add the test in an existing one not increase test time.
I chose to test both stdout & stderr streams on purpose even if, today, it's on stderr.
I was a bit hesitant to check for a shorter string. If the string changes, the test won't be doing anything useful & maybe just checking for warning is better even if not targeted (thus could report unrelated warnings in the future).

[joerick]

@mayeut i hope it's okay I added another test to this, just checking that the warning is detected by forcing it to be produced with a MACOSX_DEPLOYMENT_TARGET=10.8. It's only a 5s test.