Add a "waitFor" stanza to local dpxdt command line tool

A full command which waits for a server to start up on a given port with a timeout looks like this:

    waitFor: |
        timeout=$(($(date +%s) + 5))  # 5 second timeout
        until \
            curl --silent -I 'http://localhost:5001/' | grep '200 OK' || \
            [[ $(date +%s) -gt $timeout ]]; do
          echo 'Waiting for server to start up...'
          sleep 1
        done
    
        if [[ $(date +%s) -gt $timeout ]]; then
          echo 'Server failed to start.' 1>&2
          exit 1
        else
          echo 'Server started!'
        fi

The timeout should probably be rolled into local_pdiff.py itself.

This also adds retry logic to screenshot capture, which fixes lots of flakes.

Closes #93.