Include the tests directory to flake8 targets

[tfujiwar]

What this patch does to fix the issue.

• Include the tests directory to flake8 targets
• Fix some flake8 errors in tests

Link to any relevant issues or pull requests.

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[iizukak]

OA

[tsawada]

Readability Approval

nit: you generally don't need ./ to point to things in the CWD. It is only necessary when you want to run the path as an executable. (no need to fix)

[tfujiwar]

@tsawada Thanks for the comment!

[tfujiwar]

/ready

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