multipart_test: add namespace

Added an anonymous namespace. Putting codes in the global namespace is
generally not recommended.

Reference:
https://stackoverflow.com/questions/47861534/why-does-google-test-sample-put-tests-in-an-anonymous-namespace

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Ic141a1e71b80a47f9b9c0c5a0a73da68ee7c72d9
diff --git a/include/ut/multipart_test.cpp b/include/ut/multipart_test.cpp
index 6155e3e..aa55d02 100644
--- a/include/ut/multipart_test.cpp
+++ b/include/ut/multipart_test.cpp
@@ -9,7 +9,11 @@
 
 #include <gtest/gtest.h>
 
-class MultipartTest : public ::testing::Test
+namespace
+{
+using ::testing::Test;
+
+class MultipartTest : public Test
 {
   public:
     boost::beast::http::request<boost::beast::http::string_body> req{};
@@ -239,3 +243,4 @@
     crow::Request reqIn(req, ec);
     ASSERT_EQ(parser.parse(reqIn), ParserError::ERROR_HEADER_ENDING);
 }
+} // namespace
\ No newline at end of file