[Test] (MG_Test): cover the indirect gl_InstanceID probe and shader rewrite

BackendLoaderTest drives ProbeIndirectInstanceIdIncludesBaseInstance
(now externally linked) against a fake GLES function table: conforming
and ANGLE-style leaking drivers, the no-vertex-SSBO skip, draw-error
inconclusiveness, object cleanup, and the FillInGLESCapabilities wiring
end-to-end. SanityTest gains PromoteDrawParameterGlobalsToUniforms
cases pinning the mg_ZeroBasedInstanceID rewrite and the
last-SSBO-binding computation against a non-default binding count,
with RAII capability restoration.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

MobileGL/MG_Test/CMakeLists.txt

@@ -64,6 +64,7 @@ set(LINK_LIBRARIES
 include(GoogleTest)
 gtest_discover_tests(SanityTest DISCOVERY_TIMEOUT 30)
 
+add_subdirectory(BackendLoader)
 add_subdirectory(Buffer)
 add_subdirectory(EGLState)
 add_subdirectory(Framebuffer)