This moves a handful of the smaller tests to using the new npm mock that
uses the real actual npm object.  It also extends the testing surface
area of a few tests back down into the actual `process.spawn` that
results, instead of anything internal to the code.

Some dead code in `lib/test.js` was found during this, as well as an
instance of a module throwing a string instead of an error object.

PR-URL: #3463
Credit: @wraithgar
Close: #3463
Reviewed-by: @nlf

  * fix: properly handle top-level files when using strip
  * Avoid an unlikely but theoretically possible redos
  * WriteEntry backpressure
  * fix(unpack): always resume parsing after an entry error
  * fix(unpack): fix hang on large file on open() fail
  * fix: properly prefix hard links

 * Clear progress bar which overlays confirm prompt

  * fix: start retrying immediately, stop after 10 attempts