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.