Add comments to unit tests

Merge from Widevine repo of http://go/wvgerrit/73383

Partners have been requesting more explanation for what each test is
doing.  This is an attempt to clarify some of the tests.

Bug: 119640588
Test: unit tests
Change-Id: I270203b4e8fa7e65abb55297788e4d55856e7dcd

libwvdrmengine/oemcrypto/test/oemcrypto_test_main.cpp

@@ -12,6 +12,9 @@ static void acknowledge_cast() {
       << "==================================================================\n";
 }
 
+// This special main procedure is used instead of the standard GTest main,
+// because we need to initialize the list of features supported by the device.
+// Also, the test filter is updated based on the feature list.
 int main(int argc, char** argv) {
   ::testing::InitGoogleTest(&argc, argv);
   wvcdm::g_cutoff = wvcdm::LOG_INFO;