Tests: Add case for source files named with CONFIG genex

This is now expected to work on all generators where the test runs.
Recent ancestors of this commit fix the Visual Studio generator.

Fixes: #20648

Tests/ConfigSources/CMakeLists.txt

@@ -5,6 +5,12 @@ if(NOT _isMultiConfig AND NOT CMAKE_BUILD_TYPE)
 endif()
 project(ConfigSources CXX)
 
+# Source file(s) named with the configuration(s).
+file(GENERATE
+  OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/config_$<CONFIG>.cpp"
+  CONTENT "void config_$<CONFIG>() {}\n"
+  )
+
 # Per-config sources via INTERFACE_SOURCES.
 add_library(iface INTERFACE)
 target_sources(iface INTERFACE
@@ -21,6 +27,7 @@ add_executable(ConfigSources
   $<$<CONFIG:Debug>:main_debug.cpp>
   $<$<NOT:$<CONFIG:Debug>>:main_other.cpp>
   $<$<CONFIG:NotAConfig>:does_not_exist.cpp>
+  ${CMAKE_CURRENT_BINARY_DIR}/config_$<CONFIG>.cpp
   )
 target_link_libraries(ConfigSources iface)