Add special test for generic address space atomics (#1959)

The atomic operations are tested with generic pointers but in a way
where the compiler can infer the original pointer address space. This
commit adds tests specifically for the case where the compiler cannot
make inferences.
Test that the correct address is used with atomics when the address is
group variant and invariant.

test_conformance/generic_address_space/CMakeLists.txt

@@ -2,6 +2,7 @@ set(MODULE_NAME GENERIC_ADDRESS_SPACE)
 
 set(${MODULE_NAME}_SOURCES
     advanced_tests.cpp
+    atomic_tests.cpp
     basic_tests.cpp
     main.cpp
     stress_tests.cpp