pgsql: Add simple tests of EvalPlanQual using the isolationtester infra

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/759d9d67695783f6d04a85aba383a41c5382548c

Modified Files
--------------
src/test/isolation/README | 89 ++++++++++++++++--------
src/test/isolation/expected/eval-plan-qual.out | 51 ++++++++++++++
src/test/isolation/isolation_schedule | 1 +
src/test/isolation/isolationtester.c | 17 ++---
src/test/isolation/specs/eval-plan-qual.spec | 56 +++++++++++++++
5 files changed, 176 insertions(+), 38 deletions(-)