Fix: bt2: use of undefined variable in _VariantFieldClassWithSelectorTestCase.test_co...

Fix: bt2: use of undefined variable in _VariantFieldClassWithSelectorTestCase.test_const_append

Flake8 reports:

    ./tests/bindings/python/bt2/test_field_class.py:875:43: F821 undefined name 'str_field_class'

Fix it by defining the str_field_class variable (even though it won't
actually be used).

Reported-by: flake8
Change-Id: If17cef30ed24b6ec9d1ee25d3f6c94d491ffd8fd
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2090