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_session_tests_helper.h

@@ -26,8 +26,12 @@ public:
   // If force is true, we assert that the key loads successfully.
   void CreateWrappedRSAKey(uint32_t allowed_schemes, bool force);
 
+  // This is used to force installation of a keybox.  This overwrites the
+  // production keybox -- it does NOT use OEMCrypto_LoadTestKeybox.
   void InstallKeybox(const wvoec::WidevineKeybox& keybox, bool good);
 
+  // This loads the test keybox or the test RSA key, using LoadTestKeybox or
+  // LoadTestRSAKey as needed.
   void EnsureTestKeys();
 
   void InstallTestSessionKeys(Session* s);