add unittest_string.cc that reveals that the old implementation of pEp::constant_time_equal() was WRONG!

Unfortunately, in the new Makefile the unittests seems to be no longer built. :-/

server/unittest_string.cc

@@ -0,0 +1,41 @@
+#include <gtest/gtest.h>
+
+#include <pEp/constant_time_algo.hh>
+
+namespace {
+
+const char nullo[4] = {0,0,0,0};
+const char null_x[4] = { '\0', 'X', '\0', '\n' };
+
+const std::vector<std::string> testValuesInput =
+	{
+		{ ""                   },  // always start with the simple case ;-)
+		{ "123"                },  // some ASCII digits. Still easy.
+		{ "\n\\\b"             },  // backslash escapes for ASCII and control chars
+		{ "äöü\x80\x7f"        },  // also with some non-ASCII chars
+		
+		{ std::string(nullo, nullo+1)   },  // Yeah, 1 NUL byte
+		{ std::string(nullo, nullo+2)   },  // Yeah, 2 NUL bytes
+		{ std::string(null_x, null_x+4) },  // guess what...
+		
+		{ "EOF" }
+	};
+
+}
+
+
+class StringCompareTest : public ::testing::TestWithParam<std::string>
+{
+	// intentionally left blank for now.
+};
+
+INSTANTIATE_TEST_CASE_P(StringcompareTestInstance, StringCompareTest, testing::ValuesIn(testValuesInput) );
+
+
+TEST_P( StringCompareTest, Equal )
+{
+	const auto param = GetParam();
+	EXPECT_TRUE( pEp::constant_time_equal(param, param) );
+	EXPECT_FALSE( pEp::constant_time_equal(param, "€") );
+	EXPECT_FALSE( pEp::constant_time_equal("@@@", param) );
+}