(https://github.com/ruby/irb/pull/917)

* Use 'irbtest-' instead if 'irb-' as prefix of test files.

Otherwise IRB would mis-recognize exceptions raised in test files as
exceptions raised in IRB itself.

* Support `IRB.conf[:BACKTRACE_FILTER]``

This config allows users to customize the backtrace of exceptions raised
and displayed in IRB sessions. This is useful for filtering out library
frames from the backtrace.

IRB expects the given value to response to `call` method and return
the filtered backtrace.

https://github.com/ruby/irb/commit/6f6e87d769