Fix incorrectly-specified platform list in CMake file. (#633)

Our CMake file that sets compiler conditionals has the wrong syntax when
setting `SWT_NO_EXIT_TESTS` conditionally. Fix that. (With thanks to
@compnerd.)

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

cmake/modules/shared/CompilerSettings.cmake

@@ -20,7 +20,8 @@ add_compile_options(
 if(APPLE)
   add_compile_definitions("SWT_TARGET_OS_APPLE")
 endif()
-if(CMAKE_SYSTEM_NAME IN_LIST "iOS;watchOS;tvOS;visionOS;WASI")
+set(SWT_NO_EXIT_TESTS_LIST "iOS" "watchOS" "tvOS" "visionOS" "WASI")
+if(CMAKE_SYSTEM_NAME IN_LIST SWT_NO_EXIT_TESTS_LIST)
   add_compile_definitions("SWT_NO_EXIT_TESTS")
 endif()
 if(NOT APPLE)