nbtree _bt_set_startikey row compare test coverage

_bt_set_startikey is one notable place where nbtree's test coverage is
lacking. I never added any tests as part of my commit 7d9cd2df, which
added the ability for _bt_readpage to skip over row compare scan keys
when evaluating the scan key against every tuple on the page is
provably unnecessary.

Attached patch adds those tests now.

--
Peter Geoghegan