tests: lib: p4wq: turn of time slicing

Turns off time slicing as these tests do not expect time slicing and
previously operated without time slicing, but now fallback to the new
default of a time slice size of 20 added in e337b7b. This explicitly sets
the time slice size to 0.

Signed-off-by: Daniel Mangum <georgedanielmangum@gmail.com>

Author: hasheddan

tests/lib/p4workq/prj.conf

@@ -1,6 +1,7 @@
 CONFIG_TEST_RANDOM_GENERATOR=y
 CONFIG_ZTEST=y
 CONFIG_SCHED_DEADLINE=y
+CONFIG_TIMESLICE_SIZE=0
 CONFIG_LOG_DEFAULT_LEVEL=1
 # Test whiteboxes the wait_q and expects it to be a dlist
 CONFIG_WAITQ_SCALABLE=n