tests/frontend/buildcheckout.py: Fix bad filename issue for Windows

In !1028, we added a test specifically to test
that BuildStream correctly raises an warning when the name of an element
contains characters that are invalid on Windows. Unfortunately, we
didn't see it coming that it would make it impossible to checkout this
branch on Windows.

Fix it by generating this file, only if we are not running on Windows.

* tests/testutils/site.py: Add `IS_WINDOWS` check
* tests/frontend/buildcheckout.py: Generate file with invalid filename
  on the fly
* Remove tests/frontend/project/elements/invalid-chars.

Fixes #842.

Note that this may still cause issues on WSL when running tests on a
shared filesystem, but that seems to be a generic issue on WSL with
`os.rename`.