project: Add test for platform variables with a Generic system

Cover that `UNIX`, `LINUX`, and other host platform variables are not
defined after `project()` when `CMAKE_SYSTEM_NAME` is `Generic`.

Issue: #28200

Tests/RunCMake/project/PlatformVariablesGeneric.cmake

@@ -0,0 +1,5 @@
+foreach(var APPLE UNIX WIN32 CYGWIN MSYS BSD LINUX AIX)
+  if(DEFINED ${var})
+    message(SEND_ERROR "${var} is defined as '${${var}}' for a Generic system.")
+  endif()
+endforeach()

Tests/RunCMake/project/RunCMakeTest.cmake

@@ -35,6 +35,7 @@ run_cmake(LanguagesUnordered)
 if(RunCMake_GENERATOR MATCHES "Make|Ninja")
   run_cmake(LanguagesUsedButNotEnabled)
 endif()
+run_cmake_with_options(PlatformVariablesGeneric -DCMAKE_SYSTEM_NAME=Generic)
 run_cmake(ProjectCMP0126)
 run_cmake(ProjectCompatVersion)
 run_cmake(ProjectCompatVersion2)