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

@@ -75,6 +75,13 @@ maintainer-clean: distclean
 installcheck: all
 	./pg_isolation_regress --psqldir='$(PSQLDIR)' --inputdir=$(srcdir) --schedule=$(srcdir)/isolation_schedule
 
+# Version of the install check test that includes the prepared_transactions test
+# It only makes sense to run this if set up to use prepared transactions,
+# via the postgresql.conf.
+installcheck-prepared-txns: all
+	./pg_isolation_regress --psqldir='$(PSQLDIR)' --inputdir=$(srcdir) --schedule=$(srcdir)/isolation_schedule prepared-transactions
+
+
 # We can't support "make check" because isolationtester requires libpq, and
 # in fact (on typical platforms using shared libraries) requires libpq to
 # already be installed.  You could run "make install" and then run a check

src/test/isolation/isolation_schedule

@@ -9,4 +9,3 @@ test: ri-trigger
 test: partial-index
 test: two-ids
 test: multiple-row-versions
-test: prepared-transactions