Add isolationtester spec for old heapam.c bug

Add isolationtester spec for old heapam.c bug

In 0e5680f4737a, I fixed a bug in heapam that caused spurious deadlocks
when multiple updates concurrently attempted to modify the old version
of an updated tuple whose new version was key-share locked.  I proposed
an isolationtester spec file that reproduced the bug, but back then
isolationtester wasn't mature enough to be able to run it.  Now that
38f8bdcac498 is in the tree, we can have this spec file too.

Discussion: https://www.postgresql.org/message-id/20141212205254.GC1768%40alvh.no-ip.org