Test passing expanded-value representations to workers.

Test passing expanded-value representations to workers.

Currently, we don't have an explicit test to pass expanded-value
representations to workers, so we don't know whether it works on all kind
of platforms.  We suspect that the current code won't work on
alignment-sensitive hardware.  This commit will test that aspect and can
lead to failure on some of the buildfarm machines which we will fix in the
later commit.

Author: Tom Lane and Amit Kapila
Discussion: https://postgr.es/m/11629.1536550032@sss.pgh.pa.us