[FFmpeg-user] Concatenation of two *.mp4 files --- How?

[Virgil Stokes]

I have two mp4 files (the first (testImage.mp4 with no audio) created
 from a png file with ffmpeg [vers. N-79947-gcd244fa Copyright (c) 2000-2016
 the FFmpeg developers built with gcc 5.3.0 (GCC)]

and the second (test.mp4 width video and audio) downloaded from YouTube.

I obtained the following information on these files using
 ffprobe [vers. N-79947-gcd244fa Copyright (c) 2007-2016 the
 FFmpeg developers built with gcc 5.3.0 (GCC)]:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'testImage.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.36.100
  Duration: 00:00:10.00, start: 0.000000, bitrate: 33 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280
[SAR 4005:1184 DAR 445:74], 30 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
    Metadata:
      handler_name    : VideoHandler

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'test.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2015-10-28 06:13:56
  Duration: 00:01:35.64, start: 0.000000, bitrate: 1370 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720
[SAR 1:1 DAR 16:9], 1176 kb/s, 29.97 fps, 29.97 tbr, 30k tbn (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo,
    fltp, 192 kb/s (default)
    Metadata:
      creation_time   : 2015-10-28 06:13:56
      handler_name    : IsoMedia File Produced by Google, 5-11-2011

How can I concatenate these two files with ffmpeg?