[6.0] Fix incorrectly-specified platform list in CMake file. (#634)

**Explanation:** Correctly sets `SWT_NO_EXIT_TESTS` on platforms that
don't support them when building with CMake.
**Scope:** Building iOS, watchOS, tvOS, visionOS, WASI from OSS.
**Issue:** N/A
**Original PR:** #633
**Risk:** Low
**Testing:** Manually tested as we don't have CI using CMake yet.
**Reviewer:** @compnerd @briancroom

Author: grynspan