Rename LibcurlHttpFetcherTest

Both libcurl_http_fetcher_unittest.cc and http_fetcher_unittest.cc have
a class named "LibcurlHttpFetcherTest", and they do different things.
This is an ODR violation that took ~2 days to investigate. Fix by
renaming + moving classes to anon namespace.

Test: th
Change-Id: I2271b3563a532f225b1efa4070e098246075e8da
diff --git a/libcurl_http_fetcher_unittest.cc b/libcurl_http_fetcher_unittest.cc
index 5d67570..3543870 100644
--- a/libcurl_http_fetcher_unittest.cc
+++ b/libcurl_http_fetcher_unittest.cc
@@ -31,8 +31,8 @@
 namespace chromeos_update_engine {
 
 namespace {
+
 constexpr char kHeaderName[] = "X-Goog-Test-Header";
-}
 
 class LibcurlHttpFetcherTest : public ::testing::Test {
  protected:
@@ -48,6 +48,8 @@
   UnresolvedHostStateMachine state_machine_;
 };
 
+}  // namespace
+
 TEST_F(LibcurlHttpFetcherTest, GetEmptyHeaderValueTest) {
   const string header_value = "";
   string actual_header_value;