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>