Rewrite stacktrace test
Rewrite stacktrace test

This makes it more resilient to PHPUnit changes and more of a test,
with the drawback that any edit between the start of
'testCanGetStacktrace' and the end of 'generateBacktrace' will cause
it to fail

I think this is OK — we have similar tests in Ruby — but we can
re-evaluate if it causes issues

bugsnag/bugsnag-php,bugsnag/bugsnag-php,bugsnag/bugsnag-php