Adjust nonce flood rate in mock oemcrypto

Merge from Widevine repo of http://go/wvgerrit/43721

This CL allows the tester to change the nonce flood rate from the
default of 20.  A tester would want this value to be larger to make
tests run more quickly.

Setting the rate to 1 makes every other nonce request a flood error.
A tester wants to do this in order to verify cdm code responds to
nonce flood correctly.  Several failing oemcrypto tests have also been
corrected.

This CL changes test code only.

bug: 73607610
test: unit tests
Change-Id: I3f52ff7ea9bcc1db7bc0e010da0b64a12d3b4dd3

libwvdrmengine/oemcrypto/mock/src/oemcrypto_engine_mock.h

@@ -146,6 +146,9 @@ class CryptoEngine {
 
   virtual bool srm_blacklisted_device_attached() { return false; }
 
+  // Rate limit for nonce generation.  Default to 20 nonce/second.
+  virtual int nonce_flood_count() { return 20; }
+
   // Set destination pointer based on the output destination description.
   OEMCryptoResult SetDestination(OEMCrypto_DestBufferDesc* out_description,
                                  size_t data_length, uint8_t subsample_flags);