Add Go testdata to vendor.yml

[fgblomqvist]

This adds Go testdata folders to vendors.yml as per #4420.

[fgblomqvist]

I'm a little lost regarding how you want this tested. There doesn't seem to be any other tests for vendor.yml patterns (only one for README in test_blob#test_vendored?) but I put a test in there s since that's what the instructions say in vendor.yml. Lmk if the test isn't of the correct format (wasn't sure which function you want to be used, but sample_blob seemed to do the job, not requiring any empty dummy-files etc.).

[stale]

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[pchaigno]

LGTM!

Tests are generally not required for such changes (i.e., adding items to our vendored and documentation lists).

[fgblomqvist]

Alright, sounds good!