Merge standalone diagnostics test-registration qualification for Gex

Retain Gex fidelity and the shared diagnostics runtime while bringing in
the validated standalone-fixture registration guard.

runtime/check_test_registration.cmake

@@ -25,14 +25,13 @@
 #
 # HOW IT WORKS
 # ------------
-# The two projects here (recompiler/ and runtime/) are separate project()s with
-# separate BUILD_TESTING gates, and runtime/ additionally needs a generated BIOS
+# The recompiler, runtime and standalone playtest diagnostics projects have
+# separate test gates, and runtime/ additionally needs a generated BIOS
 # to configure — so there is no single tree in which CMake can see every
 # registered target. Rather than force them together (which would make the
-# BIOS-free recompiler tree unconfigurable for newcomers), this reads both
+# BIOS-free recompiler tree unconfigurable for newcomers), this reads their
 # CMakeLists.txt files as TEXT and compares the test files they mention against
-# the test files on disk. That works identically from either project, so both
-# call it and each independently catches an orphan in the other.
+# the test files on disk. The runtime and recompiler call it independently.
 #
 # Usage: call at the END of a CMakeLists.txt.
 #   include(${CMAKE_CURRENT_SOURCE_DIR}/check_test_registration.cmake)
@@ -54,7 +53,8 @@ function(psxrecomp_check_all_tests_registered)
     # ---- what the build systems say -------------------------------------
     set(_declared "")
     foreach(_cml "${_root}/runtime/CMakeLists.txt"
-                 "${_root}/recompiler/CMakeLists.txt")
+                 "${_root}/recompiler/CMakeLists.txt"
+                 "${_root}/runtime/tests/playtest_diagnostics/CMakeLists.txt")
         if(NOT EXISTS "${_cml}")
             continue()
         endif()