tests, base: Fixed incorrect implementation of StrTest.ToLower test.

Change-Id: I96672052f8c9da9d4f61ff0e8eed324032b1afac
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22123
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/str.test.cc

@@ -39,6 +39,8 @@
 
 #include <gtest/gtest.h>
 
+#include <cstdint>
+
 #include "base/str.hh"
 
 /*
@@ -95,7 +97,7 @@ TEST(StrTest, EatWhiteNoWhitespace)
 TEST(StrTest, ToLower)
 {
     std::string val = "gOoDbYe FOO@barr!";
-    to_lower("goodbye foo@bar!");
+    EXPECT_EQ("goodbye foo@barr!", to_lower(val));
 }
 
 /*