blockfilter: Remove sharp edge (uninitialized m_filter_type) when using the compiler-generated constructor for BlockFilter

Summary:
backport of core PR 15532 (commit 82c3b3f8e)

Note: due to BOOST_CHECK_EQUAL barfing on the new enum class uint8_t,
I've switched that test to also use just BOOST_CHECK.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: Mengerian, jasonbcox

Maniphest Tasks: T589

Differential Revision: https://reviews.bitcoinabc.org/D2879