Test MAC signature after license reload

Merge from Widevine repo of http://go/wvgerrit/78143

This verifies that the correct mac keys are used to sign a message when
generating a usage report or secure stop.

Test: unit tests on ref code and taimen
Bug: 111261264
Change-Id: I7d56a6ff0aae68a5554f7ce84d467697d1f9997b

libwvdrmengine/oemcrypto/test/oec_session_util.cpp

@@ -1359,6 +1359,9 @@ void Session::GenerateVerifyReport(const std::string& pst,
   Test_PST_Report expected(pst, status);
   ASSERT_NO_FATAL_FAILURE(VerifyReport(expected, time_license_received,
                                        time_first_decrypt, time_last_decrypt));
+  // The PST report was signed above.  Below we verify that the entire message
+  // that is sent to the server will be signed by the right mac keys.
+  ASSERT_NO_FATAL_FAILURE(VerifyClientSignature());
 }
 
 void Session::CreateOldEntry(const Test_PST_Report& report) {