Make tests independent of whether CHECKDATASIG is included in mandatory or standard flags

Summary:
Currently, in addition to testing the functioning of CHECKDATASIG,
sigopcount_tests.cpp and checkdatasig_tests.cpp implicitly test that
CHECKDATASIG is not included in standard or mandatory flags.

Change this to make the CHECKDATASIG tests independent of whether CHECKDATASIG
is included in standard or mandatory flags. To maintain test coverage, also
add a test to explicitly test whether CHECKDATASIG is included in standard or
mandatory flags.

Test Plan:
make check - passes
Try adding CHECKDATASIG to standard flags - make check - test fails.
Try adding CHECKDATASIG to mandatory flags - make check - test fails.

Reviewers: deadalnix, Fabien, #bitcoin_abc, markblundeberg

Reviewed By: deadalnix, #bitcoin_abc

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3063