Add some test coverage of EvalPlanQual with non-locked tables.

A Salesforce colleague of mine griped that the regression tests don't
exercise EvalPlanQualFetchRowMarks() and allied routines.  Which is
a fair complaint.  Add test cases that go through the REFERENCE and COPY
code paths.  Unfortunately we don't have sufficient infrastructure right
now to exercise the FDW code path in the isolation tests, but this is
surely better than before.