prune: Handle testdata directories

[sigma]

What does this do / why do we need it?

"testdata" is special, as documented in commit
golang/go@2f9eeea

Therefore such a directory can be discarded when test files are.

This should fix #1580

What should your reviewer look out for in this PR?

Do you need help or clarification on anything?

Which issue(s) does this PR fix?

[jmank88]

This seems straightforward, and I'm surprised it wasn't already included. Was there a specific reason not to remove testdata dirs? @sdboyer @ibrasho

Do we have to consider legal/license files buried under testdata?

[sigma]

I guess it was coincidentally covered by non-go in most cases, so maybe was not too visible?

As for "legal" files, I personally don't like the amount of currently preserved files in otherwise empty directories. We regularly end up with relatively deep hierarchies containing just a license file that does not apply to anything still present, which is confusing at best.
I'd be happier with a rule like "preserve legal files in directories that contain a used package" (when unused packages are pruned of course) or something like that, but that's definitely a separate discussion :)
In any case, I don't think we should see cases where legal files in testdata would cover more than a subset of the test data itself (granted, anything's possible)

[sdboyer]

It is allowed by the compiler to import directly from testdata dirs. Same as it's allowed to import from a _* or .*-shaped directory - but all three of these shapes are ignored, by default, by go list.

That's why we can't do this. The fix for #1580 is to work out the issue with the symlinks.