Previously, there was at most one "hard error", as opposite to multiple
"soft errors". This was done to preserve the historic behavior at the
time of introducing multiple `TestInfo.errors`.
With this change, every user callback that is executed `withRunnable()`
can throw an error and/or timeout, and both of these will end up in
`TestInfo.errors`.
Additionally, there is at most one "unhandled exception" error, to avoid
flooding the report with mass failures.
Drive-by: remove boolean arguments from `_failWithError()`.
Fixes#29876.
3e73a6ce69