Improve regression test coverage of expand_tuple().

Improve regression test coverage of expand_tuple().

I was dissatisfied with the code coverage report for expand_tuple() in the
wake of commit 7c44c46de: while better than no coverage at all, it was
still not exercising the core function of inserting out-of-line default
values, nor was the HeapTuple-output path covered.  So far as I can find,
the only code path that reaches the latter at present is EvalPlanQual
fetches for non-locked tables.  Hence, extend eval-plan-qual.spec to
test cases where out-of-line defaults must be inserted into a tuple
fetched from a non-locked table.

Discussion: https://postgr.es/m/87woxi24uw.fsf@ansel.ydns.eu