[Tests] Require exact match in assert_start_raises_init_eror()

Summary:

This allows for exact testing, since the match can be specified with a
strict regex.
Internal details (such as exact formatting of the error message) can
still be fuzzed away by regex wildcards.

Also include later fixes:

• PR12806 for feature_blocksdir
• PR12810 for a typo in assert_raises_init_error()

Backport of core PR12718, PR12806 and PR12810
https://github.com/bitcoin/bitcoin/pull/12718/files
https://github.com/bitcoin/bitcoin/pull/12806/files
https://github.com/bitcoin/bitcoin/pull/12810/files

Fixes T568
Fixes T648

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, deadalnix, markblundeberg

Maniphest Tasks: T568, T648

Differential Revision: https://reviews.bitcoinabc.org/D3157