tests: classify some ivshmem tests as slow

Some tests may take long to run, move them under g_test_slow()
condition.

The 5s timeout for the "server" test will have to be adjusted to the worst
known time (for the records, it takes ~0.2s on my host). The "pair"
test takes ~1.7, a quickest version could be implemented.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1447326618-11686-1-git-send-email-marcandre.lureau@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>