parserTests: Don't start mock server if we just want --help

Change-Id: I4449aec2fa56c1ccf2e9653d8eba18e649ed577f
Replace optimist with yargs

The optimist package is no longer maintained upstream.  Replace it with
yargs, which is a maintained fork.

Change-Id: I44d12ec20919f2fcbb1eda1082c03789742d5cd2