ashmem_test: Rewrite fork tests to use fork() explicitly

The fork tests use the ASSERT_EXIT() macro to spawn a child process
that maniuplates ashmem buffers. However, using ASSERT_EXIT() makes
the child process creation non-obvious.

Therefore, make the tests easier to understand by using fork()
explicitly to capture what is going on in each test.

No functional change.

Bug: 390672531
Change-Id: I84d3c17d229a1036030eea40e159664b8eeb3af7
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>