tests: kernel: usage: turn off time slicing in thread runtime stats tests

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>