tests: posix: xsi_realtime: add integration platforms to testsuite

Use qemu_x86, qemu_cortex_a53, and qemu_riscv64 as integration
test platforms for this testsuite, since both qemu_x86 and
qemu_cortex_a53 both have an MMU available, and qemu_riscv64
can execute quickly that mmu-less systems can also use a subset
of the option group (minus `mmap()`).

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>