```
I noticed that according to the doc tests.log should contain stdout as
well as stderr. But it doesn't because stdout isn't flushed. I removed
buffering completely to avoid having to call fflush twice.
```
Backport of secp256k1 PR685.
Depends on D5433.
It slightly varies from the PR by only printing the logs in the event of
a failure.
There is no need to explicitly cat the logs with the cmake build because
they are already printed by the test runner in case of failure.
Before: https://travis-ci.org/Fabcien/secp256k1/jobs/658610743
After: https://travis-ci.org/Fabcien/secp256k1/jobs/658631083