TidScan plan nodes were not systematically tested before.  These additions
raise the LOC coverage number for the basic regression tests from 52% to
92% in nodeTidscan.c, and from 60% to 93% in tidpath.c.

Andres Freund, tweaked a bit by me

Discussion: https://postgr.es/m/20170320062511.hp5qeurtxrwsvfxr@alap3.anarazel.de