Improve RUBYOPT's handling in tests

[st0012]

When debugging the issue related to https://github.com/ruby/irb/pull/919, I noticed that debugger tests don't respect the IRB version I specified in the Gemfile. This is because console tests force override the RUBYOPT env, which will remove the -rbundler/setup injected by bundler.

Additionally, if tests use run_rdbg with the rubyopt option, the RUBYOPT will be overridden yet again.

So in this commit I did 2 improvements:

1. run_rdbg should append instead of override RUBYOPT
2. If tests are executed with bundler, we also run the debugger in PTY process with bundler by appending -rbundler/setup to RUBYOPT

[launchable-app[bot]]

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Build: refs_pull_1097_merge_ec352d20ea65c331150d36bc214e196178f619ee

[st0012]

Ruby head is failing because the commit it uses doesn't include the workaround in https://github.com/ruby/irb/pull/919 yet. Once it picks a newer commit it should pass.