Wait for any output instead of default prompt on startup

[blink1073]

Summary

• Fixes startup failure when the user has a custom prompt that doesn't match prompt_regex
• Instead of self.child.expect(prompt_regex) in set_prompt (which times out with custom prompts), wait for any characters to be printed ([\s\S]+) before sending the prompt change command
• This is a reimplementation of Do no wait for the default prompt on startup #286 that fixes the root cause rather than skipping the wait entirely

Closes #286

Test plan

• Verify existing tests pass: just test
• Manually test with a kernel that uses a custom REPL prompt to confirm no startup timeout

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.95%. Comparing base (63d79b8) to head (887075e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #357   +/-   ##
=======================================
  Coverage   90.95%   90.95%           
=======================================
  Files          51       51           
  Lines        2874     2874           
  Branches      402      402           
=======================================
  Hits         2614     2614           
  Misses        181      181           
  Partials       79       79           

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