Add test for OEMCrypto_ERROR_TOO_MANY_SESSIONS

Also restore the requirement that OEMCrypto shall support at least
8 sessions.

Bug: 19641109

Merged from Widevine CDM repo:
https://widevine-internal-review.googlesource.com/#/c/13570/

Change-Id: I0aca81b9fe1f4f6505ddf91bcf399ac910b654aa

libwvdrmengine/oemcrypto/mock/src/oemcrypto_engine_mock.h

@@ -239,6 +239,12 @@ class CryptoEngine {
 
   size_t GetNumberOfOpenSessions() { return sessions_.size(); }
 
+  size_t GetMaxNumberOfSessions() {
+    // An arbitrary limit for mock implementation.
+    static const size_t kMaxSupportedOEMCryptoSessions = 64;
+    return kMaxSupportedOEMCryptoSessions;
+  }
+
   void set_current_session_(SessionContext* current) {
     current_session_ = current;
   }