Don't test using a file in docs/

[smurfix]

The docs/ directory may not be present when building, and click itself may or may not be in a src/ subdirectory.
Thus, test with a file in tests/.

[Rowlando13]

@smurfix How did you find this issue? When would you be building that you have tests but not docs?

[smurfix]

Building my asyncclick fork as a Debian package. It assembles the package in a build directory and runs the tests from there.

[Rowlando13]

Thanks for the fix and good to know!