Tests/CXXModules: support testing with custom stdlib json locations

Tests/RunCMake/CMakeLists.txt

@@ -827,6 +827,7 @@ set_property(TEST RunCMake.DependencyGraph APPEND PROPERTY LABELS "Fortran")
 add_RunCMake_test(CXXModules
   -DCMake_TEST_MODULE_COMPILATION=${CMake_TEST_MODULE_COMPILATION}
   -DCMake_TEST_MODULE_COMPILATION_RULES=${CMake_TEST_MODULE_COMPILATION_RULES}
+  -DCMake_TEST_CXX_STDLIB_MODULES_JSON=${CMake_TEST_CXX_STDLIB_MODULES_JSON}
 )
 
 # ctresalloc links against CMakeLib and CTestLib, which means it can't be built

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -1,6 +1,12 @@
 include(RunCMake)
 
-run_cmake(Inspect)
+set(stdlib_custom_json)
+if (CMake_TEST_CXX_STDLIB_MODULES_JSON)
+  list(APPEND stdlib_custom_json
+    -DCMAKE_CXX_STDLIB_MODULES_JSON=${CMake_TEST_CXX_STDLIB_MODULES_JSON})
+endif ()
+
+run_cmake(Inspect ${stdlib_custom_json})
 include("${RunCMake_BINARY_DIR}/Inspect-build/info.cmake")
 
 # Test negative cases where C++20 modules do not work.
@@ -149,6 +155,10 @@ function (run_cxx_module_test directory)
   else ()
     set(RunCMake_TEST_OPTIONS -DCMAKE_BUILD_TYPE=Debug)
   endif ()
+  if (directory MATCHES "import-std")
+    list(APPEND RunCMake_TEST_OPTIONS
+      ${stdlib_custom_json})
+  endif ()
 
   if (RunCMake_CXXModules_INSTALL)
     set(prefix "${RunCMake_BINARY_DIR}/examples/${test_name}-install")