tools/test-crash-trace: do not use fixed signal numbers

tools/test-crash-trace: do not use fixed signal numbers

The numbers of signals vary by arch. On the common arches the signals
listed here all use the same numbers, but people are likely to use
this on more fringe architectures too, so let's use symbolic names
instead.

Also the comment about gdb "hitting the same kill" didn't make sense.

The syntax is a bit baroque, but using a helper variable does not work.
Also shellcheck complains about $[ ] which would have made this more
legible.