test: refactor test-debug-prompt

• Use cleaner process.stdin.write('.exit') to exit the process rather than proc.kill().
• Move test to sequential. It uses the default port 9229. It will fail if another inspector test (or test using port 0) is already using that port. So it needs to be run sequentially rather than in parallel with other tests. (We haven't seen many failures with it yet because there aren't a lot of other inspector tests in parallel at this time.)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test