libwebsockets: add LWS_ prefix to LWS_WITHOUT_TESTAPPS=ON cmake option

Currently, libwebsockets.mk is passing -DWITHOUT_TESTAPPS=ON to disable
the building of libwebsockets test applications, but this is doing nothing
because the option is really named  LWS_WITHOUT_TESTAPPS.
This commit fixes that by using the correct option name.

Signed-off-by: Baptiste Moraine <baptiste.moraine@kapelse.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>