Various nits in sigopcount_tests.cpp and script_P2SH_tests.cpp

Summary: As per title. Use BOOST_CHECK_EQUAL instead of assertions and various style nits.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1595