subsys: testsuite: ztest framework breaks if run in cooperative mode only

[mnkp]

Describe the bug

When running a testcase with kernel configured to run in cooperative mode only, i.e. CONFIG_NUM_PREEMPT_PRIORITIES=0, the ztest framwork generates "ASSERTION FAIL" at runtime indicating invalid thread priority value.

To Reproduce

Run any testcase in cooperative mode only. E.g.

west build -b sam_e70_xplained -- -DCONFIG_ASSERT=y -DCONFIG_NUM_PREEMPT_PRIORITIES=0 tests/kernel/fifo/fifo_api/

Logs and console output

*** Booting Zephyr OS build zephyr-v2.5.0-1487-g8929b40b78e0  ***
Running test suite fifo_api
===================================================================
START - test_fifo_thread2thread
ASSERTION FAIL [((((prio)) == -1 && z_is_idle_thread_entry((entry))) || (((-1 - 1) >= ((-16 - 1))) && ((prio)) >= ((-16 - 1)) && ((prio)) <= (-1 - 1)))] @ WEST_TOPDIR/zephyr/kernel/thread.c:540
        invalid priority (-1); allowed range: -2 to -17
E: r0/a1:  0x00000004  r1/a2:  0x0000021c  r2/a3:  0x80000000
E: r3/a4:  0x00401aa5 r12/ip:  0x00000000 r14/lr:  0x004042c1
E:  xpsr:  0x41000000
E: Faulting instruction address (r15/pc): 0x00405200
E: >>> ZEPHYR FATAL ERROR 4: Kernel panic on CPU 0
E: Current thread: 0x20400470 (main)
E: Halting system

Expected behavior

ztest framework should work also in cooperative mode only.

Additional context

By default ztest thread is running at the priority -1. When the testcase is running in cooperative mode only the default ztest thread priority should be set to -2.