Avoid using a temporary file to store screenshot after a test failure

[Vlatombe]

Temporary files are created with permissions 600, differing from defined umask. For the screenshot file that is later attached as diagnostic, this can cause permission issues in some environments.

Tested by arbitrary failing a test and checking permissions set on resulting screenshot.png that can be found under target/diagnostics/{testName}/screenshot.png.
In my case 644 instead of 600.

Testing done

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[jglick]

Pity https://www.javadoc.io/doc/org.seleniumhq.selenium/selenium-api/2.52.0/org/openqa/selenium/OutputType.html has no OutputStream option. I guess one screenshot will be small enough to have negligible memory impact.