pw_string: Add tests missing from test group

Change-Id: Ifbe35c8e5c32014ffe30a7755482a7e3906414ce
diff --git a/pw_string/BUILD.gn b/pw_string/BUILD.gn
index 59d4f90..ac9666a 100644
--- a/pw_string/BUILD.gn
+++ b/pw_string/BUILD.gn
@@ -48,8 +48,10 @@
 pw_test_group("tests") {
   tests = [
     ":format_test",
+    ":string_builder_test",
     ":to_string_test",
     ":type_to_string_test",
+    ":util_test",
   ]
   group_deps = [
     "$dir_pw_preprocessor:tests",