Remove the test case added by commit fac1b470, which never actually
worked to expose the problem it claimed to test.  Replace it with
a case that does expose the problem, and also covers the SRF-not-
at-the-top deficiency repaired in 1aa8dad41.

Richard Guo, with some editorialization by me

Discussion: https://postgr.es/m/17564-c7472c2f90ef2da3@postgresql.org