Add simple tests of EvalPlanQual using the isolationtester infrastructure.

Much more could be done here, but at least now we have *some* automated
test coverage of that mechanism.  In particular this tests the writable-CTE
case reported by Phil Sorber.

In passing, remove isolationtester's arbitrary restriction on the number of
steps in a permutation list.  I used this so that a single spec file could
be used to run several related test scenarios, but there are other possible
reasons to want a step series that's not exactly a permutation.  Improve
documentation and fix a couple other nits as well.