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.

src/test/isolation/specs/eval-plan-qual.spec

@@ -0,0 +1,56 @@
+# Tests for the EvalPlanQual mechanism
+#
+# EvalPlanQual is used in READ COMMITTED isolation level to attempt to
+# re-execute UPDATE and DELETE operations against rows that were updated
+# by some concurrent transaction.
+
+setup
+{
+ CREATE TABLE accounts (accountid text PRIMARY KEY, balance numeric not null);
+ INSERT INTO accounts VALUES ('checking', 600), ('savings', 600);
+}
+
+teardown
+{
+ DROP TABLE accounts;
+}
+
+session "s1"
+setup		{ BEGIN ISOLATION LEVEL READ COMMITTED; }
+# wx1 then wx2 checks the basic case of re-fetching up-to-date values
+step "wx1"	{ UPDATE accounts SET balance = balance - 200 WHERE accountid = 'checking'; }
+# wy1 then wy2 checks the case where quals pass then fail
+step "wy1"	{ UPDATE accounts SET balance = balance + 500 WHERE accountid = 'checking'; }
+# upsert tests are to check writable-CTE cases
+step "upsert1"	{
+	WITH upsert AS
+	  (UPDATE accounts SET balance = balance + 500
+	   WHERE accountid = 'savings'
+	   RETURNING accountid)
+	INSERT INTO accounts SELECT 'savings', 500
+	  WHERE NOT EXISTS (SELECT 1 FROM upsert);
+}
+step "c1"	{ COMMIT; }
+
+session "s2"
+setup		{ BEGIN ISOLATION LEVEL READ COMMITTED; }
+step "wx2"	{ UPDATE accounts SET balance = balance + 450 WHERE accountid = 'checking'; }
+step "wy2"	{ UPDATE accounts SET balance = balance + 1000 WHERE accountid = 'checking' AND balance < 1000; }
+step "upsert2"	{
+	WITH upsert AS
+	  (UPDATE accounts SET balance = balance + 1234
+	   WHERE accountid = 'savings'
+	   RETURNING accountid)
+	INSERT INTO accounts SELECT 'savings', 1234
+	  WHERE NOT EXISTS (SELECT 1 FROM upsert);
+}
+step "c2"	{ COMMIT; }
+
+session "s3"
+setup		{ BEGIN ISOLATION LEVEL READ COMMITTED; }
+step "read"	{ SELECT * FROM accounts ORDER BY accountid; }
+teardown	{ COMMIT; }
+
+permutation "wx1" "wx2" "c1" "c2" "read"
+permutation "wy1" "wy2" "c1" "c2" "read"
+permutation "upsert1" "upsert2" "c1" "c2" "read"