Always install Test/AbstractOpenGLTester.h.

It's counterintuitive when you have to enable `WITH_GL_TESTS` just to
install the header, moreover the dependent projects have no easy way to
detect the header presence, so they just fail on preprocessor error.

src/Test/CMakeLists.txt

@@ -42,6 +42,4 @@ endif()
 set_target_properties(ResourceManagerTest PROPERTIES COMPILE_FLAGS -DCORRADE_GRACEFUL_ASSERT)
 
 # Install bootstrap header for GL tests to be used in dependent projects
-if(BUILD_GL_TESTS)
-    install(FILES AbstractOpenGLTester.h DESTINATION ${MAGNUM_INCLUDE_INSTALL_DIR}/Test)
-endif()
+install(FILES AbstractOpenGLTester.h DESTINATION ${MAGNUM_INCLUDE_INSTALL_DIR}/Test)