Remove tests from bdist

[marmitar]

As @wvxvw pointed out in #1819, nbconvert shouldn't be packaging and distributing test files in binary wheels. This PR is a simple solution for that.

[marmitar]

So both failing tests assume the test files are installed with nbconvert. Pip always install a wheel though, even when installing from a sdist, in which case the bdist is built before installing. The only solution for this is installing in editable mode, using the source files directly for that package. This would require changing the Github action test-sdist into

python -m pip install -e ${{inputs.package_spec}}

[marmitar]

Wheel got reduced by 52%.

> ls -h dist/ new/
dist/:
total 1412
564 nbconvert-7.0.0rc3-py3-none-any.whl  848 nbconvert-7.0.0rc3.tar.gz

new/:
total 1116
268 nbconvert-7.0.0rc3-py3-none-any.whl  848 nbconvert-7.0.0rc3.tar.gz

> math \(564 - 268\) / 564
0.524823

[blink1073]

Thanks!