Deactive flapping checksum isolation tests.

They've been broken for days, and prevent other tests from being
run. The plan is to revert their addition later.

Discussion: https://postgr.es/m/20180407162252.wfo5aorjrjw2n5ws@alap3.anarazel.de

src/test/isolation/isolation_schedule

@@ -75,5 +75,5 @@ test: predicate-gist
 test: predicate-gin
 # The checksum_enable suite will enable checksums for the cluster so should
 # not run before anything expecting the cluster to have checksums turned off
-test: checksum_cancel
-test: checksum_enable
+# test: checksum_cancel
+# test: checksum_enable