Inspired by [[https://github.com/bitcoin/bitcoin/pull/22229 | core#22229]] (see D13187 and D13184)
Scripted diff:
```
flynt test/functional/feature_* --transform-concats
```
Manual changes:
- replace a "{1}" with "1"
- remove explicit string conversion inside placeholders (f"{str(foo}" -> f"{foo}")
Depends on D13190