Install XrApiLayer_conformance_test_layer.json

This is missing from at least the Openxr-cts-x64 bundle, and appears to not be installed by CMake

Taken the same approach as https://github.com/KhronosGroup/OpenXR-CTS/blob/devel/src/conformance/conformance_layer/CMakeLists.txt

Without this, `validApiLayer` fails on all runtimes when using a prebuild CTS bundle.

src/conformance/conformance_test/CMakeLists.txt

@@ -238,6 +238,11 @@ set_target_properties(
     conformance_test PROPERTIES FOLDER ${CONFORMANCE_TESTS_FOLDER}
 )
 
+install(
+    FILES ${CMAKE_CURRENT_BINARY_DIR}/XrApiLayer_conformance_test_layer.json
+    DESTINATION conformance
+)
+
 install(
     TARGETS conformance_test
     LIBRARY DESTINATION conformance