Fix to set class instance variable @debug_on_failure

[kachick]

Follow a66061f

Before Ruby 3.0, uninitialized instance variable shows warning as below.

Before

$ ruby -v && bundle exec rake 2>&1 | grep 'warning' | fgrep '/lib/' | wc -l
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin20]
175

After

$ ruby -v && bundle exec rake 2>&1 | grep 'warning' | fgrep '/lib/' | wc -l
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin20]
0

[kou]

Oh... Thanks!

[kachick]

Thank you!