enforce claiming maximum coinbase amount

Claiming a slightly lesser amount does not yield the size gains
that were seen pre rct, so this closes a fingerprinting vector

tests/core_tests/chaingen_main.cpp

@@ -264,6 +264,8 @@ int main(int argc, char* argv[])
     GENERATE_AND_PLAY(gen_bp_tx_invalid_wrong_amount);
     GENERATE_AND_PLAY(gen_bp_tx_invalid_borromean_type);
 
+    GENERATE_AND_PLAY(gen_block_low_coinbase);
+
     el::Level level = (failed_tests.empty() ? el::Level::Info : el::Level::Error);
     if (!list_tests)
     {