We describe golang/testdata/ in MANIFEST.in, and that makes testdata to
be included into sdist. However testdata does not get installed.

Since, we want to be able to run pygolang tests, even on installed
package, we need the testdata files to be present there as well.

Fix it.

See https://setuptools.readthedocs.io/en/latest/setuptools.html#including-data-files
for setuptools-related details.