Fix: Generating screenshots/pdf using os.tmpdir() for Windows

[Kivol]

Since Windows does not use the temporary directory on /tmp, so the usage of os.tmpdir() is needed.

[adieuadieu]

Hi @Kivol — thank you for this! Looks good to me. Also thank you for including a test! 🥇

Notes (to myself):

• Updates Abstract the shared file-saving code between .screenshot() and .pdf() #113
• Closes Screenshot fails on Windows, if there's no c:\tmp folder #129
• Resolvable, but will conflict with Extend screenshot with specific file path in local & element by selector #137 (@elisherer)
• If we run tests over Proxy, test won't pass—can deal with it at that time

[elisherer]

@adieuadieu, actually I did both pdf and screenshot on my PR, the only thing I miss are tests, would you like us to join forces?

[adieuadieu]

@elisherer I think what we'll do is merge this PR, and then we can merge your PR #137 as it adds even more on top of what this PR adds. Effectively we'll get this PRs test, plus your contributions in #137.