The previous implementation of BacktraceSilencer#noise did not
work correctly if more than one silencer was configured --
specifically, it would only return noise which was matched by all
silencers.

The new implementation is such that anything that has been matched by
silencers is removed from the backtrace using Array#- (array
difference), ie. we now return all elements within a backtrace that
have been matched by any silencer (and are thus removed by #silence).

Fixes #11030.