Ensure CHECK_TEST_UNITS variable as a default value

Loic Nicollerequested to merge
fix_apply into main

What does this MR do and why?

The recent introduction of CHECK_TEST_UNITS variable as side effect, as when we are not in CI this variable is not defined and don't have any default values leading to an error (unbound variable)

This MR ensure that we have an empty default value to avoid that.

CI tests will not be relevant as we'll not go in this case

Related reference(s)

close #1977 (closed)

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