Fix the path to VERSION.txt used in CMake. (#1327)

Fixes a bug introduced in #1320.

### 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

CMakeLists.txt

@@ -19,6 +19,8 @@ if(POLICY CMP0157)
   endif()
 endif()
 
+set(SWT_SOURCE_ROOT_DIR ${CMAKE_SOURCE_DIR})
+
 project(SwiftTesting
   LANGUAGES CXX Swift)
 

Sources/TestingMacros/CMakeLists.txt

@@ -12,6 +12,8 @@ if(POLICY CMP0157)
   cmake_policy(SET CMP0157 NEW)
 endif()
 
+set(SWT_SOURCE_ROOT_DIR ${CMAKE_SOURCE_DIR}/../..)
+
 project(TestingMacros
   LANGUAGES Swift)
 

cmake/modules/shared/CompilerSettings.cmake

@@ -43,7 +43,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "WASI")
   add_compile_definitions("SWT_NO_PIPES")
 endif()
 
-file(STRINGS "../VERSION.txt" SWT_TESTING_LIBRARY_VERSION LIMIT_COUNT 1)
+file(STRINGS "${SWT_SOURCE_ROOT_DIR}/VERSION.txt" SWT_TESTING_LIBRARY_VERSION LIMIT_COUNT 1)
 if(SWT_TESTING_LIBRARY_VERSION)
   message(STATUS "Swift Testing version: ${SWT_TESTING_LIBRARY_VERSION}")
   add_compile_definitions("$<$<COMPILE_LANGUAGE:CXX>:SWT_TESTING_LIBRARY_VERSION=\"${SWT_TESTING_LIBRARY_VERSION}\">")