Skip generated files in Linguist statistics

[skip ci]
diff --git a/tests/expectations/tests/.gitattributes b/tests/expectations/tests/.gitattributes
new file mode 100644
index 0000000..9699dfa
--- /dev/null
+++ b/tests/expectations/tests/.gitattributes
@@ -0,0 +1,2 @@
+# Prevent Github Linguist from counting generated files in statistics.
+*.rs linguist-generated