intel_adsp_cavs15: use twister to run kernel testcases has no output

[KangJianX]

Describe the bug
Use twister to run kernel cases on ADSP_cavs15 still failed

To Reproduce
Steps to reproduce the behavior:

1. west build -b intel_adsp_cavs15 tests/kernel/mutex/mutex_api/
2. west sign -t rimage -p ../modules/audio/sof/zephyr/ext/rimage/build/rimage -D ../modules/audio/sof/zephyr/ext/rimage/config/ -- -k ../modules/audio/sof/keys/otc_private_key.pem
3. boards/xtensa/intel_adsp_cavs15/tools/fw_loader.py -f build/zephyr/zephyr.ri
4. See error

Logs and console output
Start firmware downloading...
Open HDA device: /dev/hda
Reset DSP...
Firmware Status Register (0xFFFFFFFF)
Boot: 0xFFFFFF (UNKNOWN)
Wait: 0x0F (UNKNOWN)
Module: 0x07
Error: 0x01
IPC CMD : 0xFFFFFFFF
IPC LEN : 0xFFFFFFFF
Booting up DSP...
Firmware Status Register (0x05000001)
Boot: 0x000001 (INIT_DONE)
Wait: 0x05 (DMA_BUFFER_FULL)
Module: 0x00
Error: 0x00
Downloading firmware...
Start firmware downloading...
Failed to receive expected status
Checking firmware status...
Firmware Status Register (0x80000007)
Boot: 0x000007 (UNKNOWN)
Wait: 0x00 (UNKNOWN)
Module: 0x00
Error: 0x01
Traceback (most recent call last):
File "/home/ztest/work/zephyrproject/zephyr/boards/xtensa/intel_adsp_cavs15/tools/fw_loader.py", line 81, in
main()
File "/home/ztest/work/zephyrproject/zephyr/boards/xtensa/intel_adsp_cavs15/tools/fw_loader.py", line 66, in main
if fw_loader.check_fw_boot_status(plat_def.BOOT_STATUS_FW_ENTERED):
File "/home/ztest/work/zephyrproject/zephyr/boards/xtensa/intel_adsp_cavs15/tools/lib/loader.py", line 133, in check_fw_boot_status
raise RuntimeError(output)
RuntimeError: Firmware Status error: Status: 0x80000007
Error Code 0x0000002F

Environment (please complete the following information):
OS: Fedora28
Toolchain: Zephyr-sdk-0.12.1
Commit ID: af67564