internal/postgres: truncate search_documents in ResetTestDB

Truncate search_documents in ResetTestDB. In the next CL, we will be
dropping the search_documents.packages FK, so the deletes will no longer
cascade.

For golang/go#39629

Change-Id: I76529ad345e8829fdd327ebcdc8b59f0b3d68875
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/262774
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/postgres/test_helper.go b/internal/postgres/test_helper.go
index 779cf94..4760340 100644
--- a/internal/postgres/test_helper.go
+++ b/internal/postgres/test_helper.go
@@ -103,6 +103,7 @@
 	if err := db.db.Transact(ctx, sql.LevelDefault, func(tx *database.DB) error {
 		if _, err := tx.Exec(ctx, `
 			TRUNCATE modules CASCADE;
+			TRUNCATE search_documents;
 			TRUNCATE version_map;
 			TRUNCATE imports_unique;`); err != nil {
 			return err