tests: kernel: set timeslice size to 0 in tests that do not exercise it

Sets CONFIG_TIMESLICE_SIZE=0 for tests that previously expected to run
without timeslicing enabled, and now fail with the default
CONFIG_TIMESLICE_SIZE=20. Note that timeslicing functionality is still
included, but no actual timeslicing is performed.

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