channeldb/migtest: log migration failure instead of failing

This line was incorrectly moved when the migtest package was created for
migration 12. This PR introduces a negative test for CreateTLB which
surfaced this.

channeldb/migtest/migtest.go

@@ -74,7 +74,7 @@ func ApplyMigration(t *testing.T,
 	// Apply migration.
 	err = kvdb.Update(cdb, migrationFunc)
 	if err != nil {
-		t.Fatal(err)
+		t.Logf("migration error: %v", err)
 	}
 }