test: add test for building with unicode filenames

[bpabel]

Added an explicit test for building against unicode filenames that I neglected to add with the last PR.

[mohdzahidabdullah]

Added an explicit test for building against unicode filenames that I neglected to add with the last PR.

https://github.com/pypa/flit/pull/355#issue-638291555

[takluyver]

Let's see how this goes with the current test pipeline.

[takluyver]

Still no dice:

   E               FileNotFoundError: [WinError 2] The system cannot find the file specified: 'C:\\Users\\runneradmin\\AppData\\Local\\Temp\\pytest-of-unknown\\pytest-0\\test_build_package_with_unicod0\\packageunicode\\packageunicode\\Noël.jpg'