Ensure unicode for subprocess output

[nmuesch]

What does this PR do?

Runs ensure_unicode on the result of get_subprocess_output before returning it.

Motivation

When porting checks to Python3, we do a lot of literal string comparison between some hardcoded expected strings and the output from get_subprocess_output. Due to this and other similar scenarios, its expected/required that the output from get_subprocess_output is an appropriate non bytes format. This uses the ensure_unicode helper function to perform this action for us.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes