Because the latest debug gem now require Ruby >= 2.7, if we require `debug`
in older Rubies, it'll require a very outdated debug lib instead.

Requiring `debug` activates the debugger even when no breakpoints are hit.
Although the chance is low, it may slow down the test suite and/or cause
unexpected behavior.

This change requires `debug/prelude` instead of `debug` to avoid the
above issue while still allowing the use of breakpoints.