Refactor wallet tests as built by cmake

Summary: The previous build would add them as a library, but this actually do not work as the test get pruned by the linker before boost has the opportunity to do anything with them. Now the test are directly built in the test_bitcoin executable.

Test Plan:

ninja
./src/test/test_bitcoin

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1247