selftests: don't kill child immediately in get_metadata() test

This this test forks a child, and then the parent waits for a write() to a
pipe signalling the child is ready to be attached to. If something in the
child ASSERTs before it does this write, the test will hang waiting for it.
Instead, let's EXPECT, so that execution continues until we do the write.
Any failure after that is fine and can ASSERT.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>