Issue 6221 - Add native tests.

test/compiled/String.cpp

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
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  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #include <string>
 #include "gtest/gtest.h"
 #include "compiled/String.h"
 
-TEST(TestString, DependentString)
+TEST(TestString, constructInvalidDependentString)
 {
   DependentString s;
   EXPECT_TRUE(s.is_invalid());
 
   DependentString s2(s);
   EXPECT_TRUE(s2.is_invalid());
 }
 
-TEST(TestString, OwnedString)

[sergei, 2018/01/29 13:38:44]

It's not important for this change set but in general I would rather prefer
something like
TEST(OwnedString, ConstructingInvalidString)

[hub, 2018/01/29 16:57:46]

Done.

+TEST(TestString, constructInvalidOwnedString)
 {
   OwnedString s;
   EXPECT_TRUE(s.is_invalid());
 
   // Valid string
   OwnedString s2(2);
   EXPECT_FALSE(s2.is_invalid());
 
   // Ensure we still have an invalid string.
   OwnedString s3(s);
   EXPECT_TRUE(s3.is_invalid());
 
   // Empty valid string lead to valid string.
   OwnedString s4(u""_str);
   EXPECT_FALSE(s4.is_invalid());
 }