gitattributes: allow diff of test data

[adamjstewart]

This allows git diff to show changes to text files in tests/data.

Previously, we treated all test files as binary (equivalent to -text -diff). This resulted in useless diffs like:

$ git diff
diff --git a/tests/data/cowc_counting/data.py b/tests/data/cowc_counting/data.py
index b612f92..c5ef84a 100755
Binary files a/tests/data/cowc_counting/data.py and b/tests/data/cowc_counting/data.py differ

However, -text is sufficient to prevent line endings from being changed (necessary for stable cross-platform checksums). Now, all text files diff properly. I also added back line ending changes to data.py since we never need to checksum that.