tests: add file-write-read test

This test exhibits a POSIX behaviour regarding switching between write
and read. It's undefined result if the application doesn't ensure a
flush between the two operations (with glibc, the flush can be implicit
when the buffer size is relatively small). The previous commit fixes
this test.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1210246

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
