ashmem-test: Add tests to get a buffer's name via the GET_NAME ioctl cmd

Add tests to ensure that ashmem and memfd handle names that are as long
as their respective maximum lengths. Also, expose __ashmem_open()
internally to ensure that an ashmem fd is obtained.

Bug: 411723589
Bug: 411741996
Change-Id: Idf13335b9f423603101ab0c6e611164eda289df5
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>