test: wait for prompt before initial break in debugger tests

[inoway46]

This fixes flakiness around waitForInitialBreak() in debugger tests.

waitForInitialBreak() can be flaky during startup because the CLI may already attach and print debug> before the initial break ... in output arrives.

Example CI log: https://github.com/nodejs/node/actions/runs/23657399006

    '< For help, see: https://nodejs.org/en/docs/inspector\n' +
    '< \n' +
    'connecting to 127.0.0.1:54872 ...\n' +
    ' ok\n' +
    '\n' +
    '\n' +
    '< Debugger attached.\n' +
    '< \n' +
    '\n' +
    'debug> '

In some CI runs, tests end up waiting on that break message while startup output is still racing.
There are similar flakes in other tests, but this PR focuses on the failures seen in the past week.

Recent CI failures:

• test/parallel/test-debugger-backtrace.js:20
  • https://github.com/nodejs/node/actions/runs/23657399006
• test/parallel/test-debugger-break.js:17
  • https://github.com/nodejs/node/actions/runs/23689423627
• test/parallel/test-debugger-clear-breakpoints.js:23
  • https://github.com/nodejs/node/actions/runs/23675940885
• test/parallel/test-debugger-sb-before-load.js:23
  • https://github.com/nodejs/node/actions/runs/23697732704

Refs: #61762

[inoway46]

This test seems the most frequently failed in the past week.
https://github.com/nodejs/node/actions/runs/23689423627
https://github.com/nodejs/node/actions/runs/23524571217
https://github.com/nodejs/node/actions/runs/23362176847

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.70%. Comparing base (cc96741) to head (9f7ff44).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62489      +/-   ##
==========================================
- Coverage   91.55%   89.70%   -1.85%     
==========================================
  Files         351      692     +341     
  Lines      147653   213986   +66333     
  Branches    23224    41050   +17826     
==========================================
+ Hits       135179   191963   +56784     
- Misses      12217    14088    +1871     
- Partials      257     7935    +7678     

see 462 files with indirect coverage changes

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