Tests: Move cache variable help string test

Even though it doesn't necessarily reuse infrastructure, the other
cases in `RunCMake.CacheNewlines` are covering more similar code to
this case than those in `RunCMake.Configure`.

Tests/RunCMake/CacheNewline/RunCMakeTest.cmake

@@ -14,3 +14,9 @@ endfunction()
 run_cmake_cache_newline_test(CacheNewlines)
 run_cmake_cache_newline_test(CacheStartsWithNewline)
 run_cmake_cache_newline_test(CacheSingleNewline)
+
+block()
+  run_cmake(CacheVarHelpString)
+  set(RunCMake_TEST_NO_CLEAN 1)
+  run_cmake(CacheVarHelpString)
+endblock()

Tests/RunCMake/Configure/RunCMakeTest.cmake

@@ -73,12 +73,6 @@ if(NOT RunCMake_GENERATOR MATCHES "^Ninja Multi-Config$")
   run_cmake(NoCMAKE_DEFAULT_CONFIGS)
 endif()
 
-block()
-  run_cmake(CacheVarHelpString)
-  set(RunCMake_TEST_NO_CLEAN 1)
-  run_cmake(CacheVarHelpString)
-endblock()
-
 if(NOT CMAKE_HOST_WIN32)
   block()
     # Test a non-writable build directory.