Remove prepared transactions from main isolation test schedule.

There is no point in running this test when prepared transactions are disabled,
which is the default. New make targets that include the test are provided. This
will save some useless waste of cycles on buildfarm machines.

Backpatch to 9.1 where these tests were introduced.

src/test/isolation/Makefile

@@ -72,3 +72,13 @@ installcheck: all
 
 check: all
 	./pg_isolation_regress --temp-install=./tmp_check --inputdir=$(srcdir) --top-builddir=$(top_builddir) --schedule=$(srcdir)/isolation_schedule
+
+# Versions of the check tests that include the prepared_transactions test
+# It only makes sense to run these if set up to use prepared transactions,
+# via TEMP_CONFIG for the check case, or via the postgresql.conf for the
+# installcheck case.
+installcheck-prepared-txns: all
+	./pg_isolation_regress --psqldir='$(PSQLDIR)' --inputdir=$(srcdir) --schedule=$(srcdir)/isolation_schedule prepared-transactions
+
+check-prepared-txns: all
+	./pg_isolation_regress --temp-install=./tmp_check --inputdir=$(srcdir) --top-builddir=$(top_builddir) --schedule=$(srcdir)/isolation_schedule prepared-transactions

src/test/isolation/isolation_schedule

@@ -9,7 +9,6 @@ test: ri-trigger
 test: partial-index
 test: two-ids
 test: multiple-row-versions
-test: prepared-transactions
 test: fk-contention
 test: fk-deadlock
 test: fk-deadlock2