ashmem_test: Always include PROT_EXEC as an expected permission

Memfds can always be mapped with PROT_EXEC. There is no support for
removing that, and there is no known reason to support it. Therefore,
update the tests to always expect PROT_EXEC as part of the mapping
permissions.

Bug: 111903542
Change-Id: Ia8bd5d47e012e24045d7ea02f63fe1b15e41e933
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>