test(pluginHandlers/common): better setup & teardown

[raphinesse]

Motivation and Context

• test file setup was overly verbose
• teardown was faulty (exceptions in tests could lead to files being left behind)
• I wanted to add a regression test to the suite

Description

• use fs.outputFileSync to create files and their parent dirs
• use afterEach to reliably clean up after tests

Testing

Affected unit tests still pass

[raphinesse]

Thanks for the review, @timbru31