Native stack traces in local tests

[Simon Chopin]

Hi folks,

We've just landed a change[0] that should make life easier for those
doing native development. Up until now, in case of a crash while running
tests locally, the test harness wouldn't be able to produce a full stack
trace unless you'd though to run `./mach buildsymbols` beforehand.

With the change, the stack walker will now use the debug info in the
binaries to get the data it needs, which should remove the need for the
`buildsymbols` step.

Happy hacking!
Simon

[0]: https://bugzilla.mozilla.org/show_bug.cgi?id=1933057