XPCShellTests with npm packages

XPCShellTests that spawn a node server may use non-vendored npm packages. Make sure that the test using the npm packages has usesNPM = true added to xpshell.toml.

Adding a new dependency

  cd testing/xpcshell/moz-http2
  npm install newPackageThatIsPublishedToNPM
  # commit package.json and package-lock.json

Running a test that uses npm packages

Just run it normally. For example

./mach text netwerk/test/unit/test_node_execute_npm.js

runxpcshelltests.py will automatically install the npm packages when one of the tests in the test list has usesNPM = true