Fix nbtree high key "continuescan" row compare bug.

Fix nbtree high key "continuescan" row compare bug.

Commit 29b64d1d mishandled skipping over truncated high key attributes
during row comparisons.  The row comparison key matching loop would loop
forever when a truncated attribute was encountered for a row compare
subkey.  Fix by following the example of other code in the loop: advance
the current subkey, or break out of the loop when the last subkey is
reached.

Add test coverage for the relevant _bt_check_rowcompare() code path.
The new test case is somewhat tied to nbtree implementation details,
which isn't ideal, but seems unavoidable.