Do not crash test_all if parametrized is missing

[CLOVIS-AI]

#1422 introduced a thin wrapper around test_all.py used to provide compatibility with unittest and related test frameworks. This approach required the parametrized package, which was added to the optional dependencies. However the test_all.py script always imported this package, which led to crashes when it was not installed.

This PR edits test_all.py so it warns the user of the missing parametrized package and disables the unittest wrapper instead of crashing the script.

A much better implementation of the test framework support is underway in serpilliere#8, but it likely will not reach master for some time yet, so this PR applies a bandaid on master until then.