tests/boot-serial-test: Add a test for the moxiesim machine

Now that moxiesim supports the -bios parameter, we can check this machine
in the boot-serial tester, too, by supplying a mini bios that only writes
'T' characters to the UART.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1512031988-32490-7-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
