tests/Makefile: Only run vhost-user-test on Linux

vhost-user-test uses the linux/vhost.h header, so it must only be
enabled if CONFIG_LINUX is defined. (Previously it was enabled
for CONFIG_POSIX, which broke 'make check' on MacOSX.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>