Add cygwin support for ctest

[Berrysoft]

Description

From JohnTitor/ctest2#70

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

r? @tgross35

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[tgross35]

It will be a few days before I get the first new release of ctest published but this LGTM.