[evlatests] leap second narrative

[Ken Sowinski]

It is probably worth recording how the VLA repsonded to the leap second.
Here is a report based on looking at station board timing, d10 and the
fringe display.

1.  A few seconds before midnight phases started slewing.  This was
     probably a result of the NTP leap second implementation anticipating
     the discontinuity differently in mchost and the station boards.

2.  At midnight the station boards reported a one second time offset.
     Phases continued stable, but different from well before midnight.

3.  At one minute after midnight the station boards reset their idea
     of time to account for the leap second.  Fringes went away.

4.  About 13 seconds later fringes returned, with the same phase
     as before midnight.

When a new file was started both L350s were found to have the correct
value for IAT - UTC, 35 seconds.  The magic file deformatters use to
determine IAT - UTC has been updated.  With any luck no bad things
will happen at local midnight tonight, or at UTC midnight tomorrow.