```
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