Only install/test if PROJECT_IS_TOP_LEVEL

Further remove installing glslangtests. There isn't a need to do
that. glslangtests isn't a deliverable.

glslang/OSDependent/Unix/CMakeLists.txt

@@ -40,7 +40,7 @@ set(THREADS_PREFER_PTHREAD_FLAG ON)
 find_package(Threads REQUIRED)
 target_link_libraries(OSDependent Threads::Threads)
 
-if(ENABLE_GLSLANG_INSTALL AND NOT BUILD_SHARED_LIBS)
+if(PROJECT_IS_TOP_LEVEL AND NOT BUILD_SHARED_LIBS)
     install(TARGETS OSDependent EXPORT glslang-targets)
 
     # Backward compatibility

glslang/OSDependent/Windows/CMakeLists.txt

@@ -51,7 +51,7 @@ if(WIN32)
     source_group("Source" FILES ${SOURCES})
 endif()
 
-if(ENABLE_GLSLANG_INSTALL)
+if(PROJECT_IS_TOP_LEVEL)
     install(TARGETS OSDependent EXPORT glslang-targets)
 
     # Backward compatibility