tests: cpp: cxx: Add qemu_cortex_a53 as integration platform

This commit adds the `qemu_cortex_a53`, which is an MMU-based platform,
as an integration platform for the C++ subsystem tests.

This ensures that the `test_global_static_ctor_dynmem` test, which
verifies that the dynamic memory allocation service is functional
during the global static object constructor invocation, is tested on
an MMU-based platform, which may have a different libc heap
initialisation path.

In addition to the above, this increases the overall test coverage
ensuring that the C++ subsystem is functional on an MMU-based platform
in general.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>