chore: Separate stderr from stdout in the tests

[nieomylnieja]

Motivation

Currently, whenever sloctl logs something to stderr, like Retrying logs, it will fail the tests because assert_output is checking $output variable set by run function of bats framework. The $output contains both stderr and stdout. Fortunately, there's a flag for run --> --separate-stderr which does the job and separates both outputs into different variables.

[skrolikiewicz]

👍