Run checkdatasig_tests on a larger set of flags
Run checkdatasig_tests on a larger set of flags

Summary: As per title. Flags go up to 18, not 17.

Test Plan:
  make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2028

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