Add tests/data directory to gitattributes

This is to prevent a fresh checkout to show that test data file
tests/data/mongodb-sample-result.txt (which has a single CRLF line break
among other normal line breaks) is modified.

Tested by pushing this commit to my forked repository, checking it out
with `git clone` and confirming it would not show any conflicts right
away.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

.gitattributes

@@ -3,3 +3,4 @@
 
 # Don't let git change end of line characters in the data directory.
 perfkitbenchmarker/data/* -text
+tests/data/* -text