tests: cleanup ptimer-test

1) ptimer-test is not a qtest---it runs the ptimer.c code directly in the
ptimer-test process

2) ptimer-test has its own stubs file, so there is no need to add more
stubs to stubs/vmstate.c

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>