[tests] Fix checkinputs_test to work under magnetic anomaly

Summary:
As per title. Add CTOR ordering to test block construction. Use NULLDUMMY
instead of CLEANSTACK for checking consensus vs non-consensus script cache behavior.

Test Plan:

make -j10 VERBOSE=1 check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2092