flake8 E741 variable name warning

[behlendorf]

Motivation and Context

Resolve new flake8 warning observed in the CI with updated version:

http://build.zfsonlinux.org/builders/Ubuntu%2018.04%20x86_64%20%28STYLE%29/builds/2257/steps/shell_2/logs/stdio

Description

Update the zts-report.py script to conform to the flake8 E741 rule.

"Variables named I, O, and l can be very hard to read. This is
because the letter I and the letter l are easily confused, and
the letter O and the number 0 can be easily confused."

• https://www.flake8rules.com/rules/E741.html

How Has This Been Tested?

Locally running make checkstyle.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.