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 <honscheid@google.com>