The import path changed from "." to "src" at the same time.

Previously "PlayLog()" and "HealthLog()" were used as input for the POST
request's body.
But the JSON serialization of both instances is empty (for unknown
reasons).  Thus the POST requests failed, due to missing required fields.

The previous code intended to silence the most relevant source of log noise
("connexion.operations"), but due to the missing trailing "s", it failed
to do so.
Now all debug messages emitted during a test are silenced.

Somewhen two years ago the tests were disabled in the CI.
Since then the test were failing.

The environment should be configured outside of the application.

* `/usr/bin/env` is not necessary: `python3` is just the same
  (`/usr/bin/env python3` is helpful in a shebang, since an absolute
  path is required there)
* use the variable `$PYTHON_EXEC`: it was introduced recently, but not
  used for now