tests: posix: common: mitigate warning for qemu_arc_em

Previously, twister would fail due to a warning being escalated to
an error because CONFIG_NET_TEST was set.

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

tests/posix/common/prj.conf

@@ -24,7 +24,3 @@ CONFIG_XSI_SYSTEM_LOGGING=y
 
 # for sched_get_priority_min(), sched_get_priority_max()
 CONFIG_POSIX_PRIORITY_SCHEDULING=y
-
-# for networking
-CONFIG_NET_TEST=y
-CONFIG_TEST_RANDOM_GENERATOR=y