testccl/workload/schemachange: skip random schema test

This is very flakey. Some of it is due to #87672. Some of it was due to #85677.
There are some issues with inserts which need to be fixed. Until this
stabilizes, it's causing problems.

Along the way, I'm marking #78478 as a GA blocker so we do actually fix it.

Release note: None

pkg/ccl/testccl/workload/schemachange/BUILD.bazel

@@ -20,6 +20,7 @@ go_test(
         "//pkg/security/securitytest",
         "//pkg/server",
         "//pkg/testutils/serverutils",
+        "//pkg/testutils/skip",
         "//pkg/testutils/sqlutils",
         "//pkg/testutils/testcluster",
         "//pkg/util/leaktest",

pkg/ccl/testccl/workload/schemachange/schema_change_external_test.go

@@ -21,6 +21,7 @@ import (
 	_ "github.com/cockroachdb/cockroach/pkg/ccl"
 	"github.com/cockroachdb/cockroach/pkg/ccl/multiregionccl/multiregionccltestutils"
 	"github.com/cockroachdb/cockroach/pkg/ccl/utilccl"
+	"github.com/cockroachdb/cockroach/pkg/testutils/skip"
 	"github.com/cockroachdb/cockroach/pkg/testutils/sqlutils"
 	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
 	"github.com/cockroachdb/cockroach/pkg/workload"
@@ -33,6 +34,7 @@ import (
 func TestWorkload(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer utilccl.TestingEnableEnterprise()()
+	skip.WithIssue(t, 78478)
 
 	dir := t.TempDir()
 	ctx := context.Background()