Add a test for GetCacheSize in coins_tests.cpp

Summary:
As per title. more tests == better .

Depends on D515

Test Plan:

make check

Reviewers: freetrader, CCulianu, #bitcoin_abc, zquestz, schancel

Reviewed By: #bitcoin_abc, zquestz, schancel

Subscribers: schancel, zquestz

Differential Revision: https://reviews.bitcoinabc.org/D517