roachtest: display diff when failing system upgrade test

Before this change, failures would not show a diff like:

```
Diff:
@@ -29,11 +29,11 @@
 	"lastUpdated" TIMESTAMP NOT NULL DEFAULT now():::TIMESTAMP,
 	"valueType" STRING NULL,
 	CONSTRAINT "primary" PRIMARY KEY (name ASC),
 	FAMILY "fam_0_name_value_lastUpdated_valueType" (name, value, "lastUpdated", "valueType")
 );
-CREATE SEQUENCE public.descriptor_id_seq MINVALUE 1 MAXVALUE 9223372036854775807 INCREMENT 1 START 1;
+CREATE SEQUENCE public.descriptor_id_seq MINVALUE 1 MAXVALUE 9223372036854775807 INCREMENT 1 START 104;
 CREATE TABLE public.tenants (
 	id INT8 NOT NULL,
 	active BOOL NOT NULL DEFAULT true,
 	info BYTES NULL,
 	name STRING NULL AS (crdb_internal.pb_to_json('cockroach.sql.sqlbase.TenantInfo':::STRING, info)->>'name':::STRING) VIRTUAL,
----

```

Release note: None

pkg/cmd/roachtest/tests/BUILD.bazel

@@ -230,6 +230,7 @@ go_library(
         "@com_github_kr_pretty//:pretty",
         "@com_github_lib_pq//:pq",
         "@com_github_montanaflynn_stats//:stats",
+        "@com_github_pmezard_go_difflib//difflib",
         "@com_github_prometheus_client_golang//api",
         "@com_github_prometheus_client_golang//api/prometheus/v1:prometheus",
         "@com_github_prometheus_common//model",

pkg/cmd/roachtest/tests/validate_system_schema_after_version_upgrade.go

@@ -20,6 +20,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/cmd/roachtest/registry"
 	"github.com/cockroachdb/cockroach/pkg/cmd/roachtest/test"
 	"github.com/cockroachdb/cockroach/pkg/testutils/sqlutils"
+	"github.com/pmezard/go-difflib/difflib"
 )
 
 func registerValidateSystemSchemaAfterVersionUpgrade(r registry.Registry) {
@@ -85,8 +86,18 @@ func registerValidateSystemSchemaAfterVersionUpgrade(r registry.Registry) {
 			validateEquivalenceStep := func(str1, str2 *string) versionStep {
 				return func(ctx context.Context, t test.Test, u *versionUpgradeTest) {
 					if *str1 != *str2 {
-						t.Fatal("After upgrading, `USE system; SHOW CREATE ALL TABLES;` " +
-							"does not match expected output after version upgrade.\n")
+						diff, diffErr := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
+							A:       difflib.SplitLines(*str1),
+							B:       difflib.SplitLines(*str2),
+							Context: 5,
+						})
+						if diffErr != nil {
+							diff = diffErr.Error()
+							t.Errorf("failed to produce diff: %v", diffErr)
+						}
+						t.Fatalf("After upgrading, `USE system; SHOW CREATE ALL TABLES;` "+
+							"does not match expected output after version upgrade."+
+							"\nDiff:\n%s", diff)
 					}
 					t.L().Printf("validating succeeded:\n%v", *str1)
 				}