tests: Stop using nyc

There are some problems with nyc:
  * The coverage numbers aren't useful in our case because most of the
    code is executed outside the test process (the test code is mostly
    API client logic).
  * nyc messes with line numbers, which makes it much harder to debug
    problems.
  * We're seeing frequent SIGABRT crashes while nyc is printing the
    results table. I'm not sure if nyc is the cause of the crashes, or
    if it's making a race condition worse, or if the crashes have
    nothing to do with nyc, but we don't lose much by removing it so
    we might as well see if the crash frequency improves.