Isolation provider tests done in tests/test_base.py and had
pytest.mark.parameterize() for each isolation provider. This logic
would not work well when we had test that diverge. We could have marked
each one as compatible with one provider or another, but in the end it
turned out to be better to have the common ones in a base class and
the divergent ones in each.
NOTE: this has a strange side-effect: inherited test classes need to
have imports for all of the fixtures even if they are not explictly used
Improve the `parse_progress()` method of the container isolation
provider in the following ways:
1. Make sure that the fields of the progress report have the expected
type.
2. In case of a JSON parsing error, sanitize the invalid string so that
it doesn't contain escape sequences, or the user considers it as
trusted.
