bt2: rename test with conflicting name in test_trace.py

bt2: rename test with conflicting name in test_trace.py

The second test named `test_env_iter` seems to be testing iteration of a
const environment object, so rename it to `test_const_env_iter`.

Reported-by: flake8
Change-Id: I614756daa511b9c66b031328f11ad77053ab9fc5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2084
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>