[rpc] Add a unittest for IsDeprecatedRPCEnabled

Summary: As per title. This is in preparation for re-enabling the estimatefee rpc.

Test Plan:

make src/test/test_bitcoin && src/test/test_bitcoin --run_test=server_tests/server_IsDeprecatedRPCEnabled

Reviewers: #bitcoin_abc, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, Fabien, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2013