ztest: bug: Add friendly name helper function  for all phases
ztest: bug: Add friendly name helper function  for all phases

The `handle_signal()` function in the new ztest API (`ztest_new.c`) uses
an array of strings to lookup a friendly name for each test phase, but
the array only has three elements despite there being six test phases.
This can lead to an out-of-bounds issue. Replace the array with a helper
function and switch statement.

Signed-off-by: Tristan Honscheid <1e17951ff5ffc8afe254954c03de64afef62fe52@google.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr