added clock timing test

[hayguen]

• pigpio.h/.c: added API function for the test:
  test duration 50 ms, tolerance 5%
• pigpiod: added option '-T' to (de)activate test, default: ON
• added independent test program x_pigpio_clock,
  testing both clocks
• amended README
• Makefile: added targets 'test' and 'test-really'

[hayguen]

this patch is for "solving" #331

i'm unsure merge should go to master or develop?

i tested on Pi 3B+ and Pi 1.

[guymcswain]

@hayguen ,
First let me express my appreciation for the work you have done in this area. I think it has been a valuable learning experience for you and hopefully, with appropriate changes to the repo, will help other users of the library. That said, I need to spend time to fully evaluate what 'appropriate changes' need to be made. I don't have the time right now but I should be freed up by the end of March - I'm currently backlogged with two releases.

I'll leave you with this thought for now until I can get the bandwidth on this PR: Should more effort be put into improving the documentation on the conflicts arising from the use of audio and pigpio, if this is the root problem being addressed. As a case in point, I don't think users understand that by plugging in a an HDMI monitor they are in fact configuring the audio subsystem on the Raspberry Pi. And after users grok the fact they have indeed configured audio, a more stern caution that use of audio with pigpio requires an advanced knowledge of the library, .

[guymcswain]

@hayguen ,
Please makex_pigpio_clock.c a stand-alone utility. I don't see the value of having a test that is integrated with all the associated file changes for such a simple diagnostic.

Also, please select develop as the base branch to merge against. Thanks.