scmigrate: ensure target key is correctly renamed (backport #8276) (#8280)

Prior to v0.35, the keys for seen-commit records included the applicable
height.  In v0.35 and beyond, we only keep the record for the latest height,
and its key does not include the height.

Update the seen-commit migration to ensure that the record we retain after
migration is correctly renamed to omit the height from its key.

Update the test cases to check for this condition after migrating.

(cherry picked from commit f3858e52de)

scripts/scmigrate/migrate_test.go

@@ -1,6 +1,7 @@
 package scmigrate
 
 import (
+	"bytes"
 	"context"
 	"math/rand"
 	"testing"
@@ -104,15 +105,6 @@ func TestMigrations(t *testing.T) {
 			assertWellOrderedMigrations(t, testData)
 		})
 	})
-	t.Run("GetMigrationsToDelete", func(t *testing.T) {
-		for i := 1; i < 100; i++ {
-			data := appendRandomMigrations([]toMigrate{}, i)
-			toMigrate := getMigrationsToDelete(data)
-			if len(data) != len(toMigrate)+1 {
-				t.Fatalf("migration prep did not save one document [original=%d migrations=%d]", len(data), len(toMigrate))
-			}
-		}
-	})
 	t.Run("InvalidMigrations", func(t *testing.T) {
 		if _, err := makeToMigrate(nil); err == nil {
 			t.Fatal("should error for nil migrations")
@@ -156,18 +148,24 @@ func TestMigrations(t *testing.T) {
 			// safe to rerun
 			t.Run(test, func(t *testing.T) {
 				if err := Migrate(ctx, db); err != nil {
-					t.Fatal(err)
+					t.Fatalf("Migration failed: %v", err)
 				}
 
 				post, err := getAllSeenCommits(ctx, db)
 				if err != nil {
-					t.Fatal(err)
+					t.Fatalf("Fetching seen commits: %v", err)
 				}
+
 				if len(post) != 1 {
-					t.Fatal("migration was not successful")
+					t.Fatalf("Wrong number of commits: got %d, wanted 1", len(post))
 				}
-				if post[0].commit.Height != latestHeight {
-					t.Fatal("migration did not save correct document")
+
+				wantKey := makeKeyFromPrefix(prefixSeenCommit)
+				if !bytes.Equal(post[0].key, wantKey) {
+					t.Errorf("Seen commit key: got %x, want %x", post[0].key, wantKey)
+				}
+				if got := post[0].commit.Height; got != latestHeight {
+					t.Fatalf("Wrong commit height after migration: got %d, wanted %d", got, latestHeight)
 				}
 			})
 		}