The file dune.inc is linguist-generated

This PR hides the file `tests/dune.inc` in statistics and diff as
it is generated by `gentest.ml` and modifying only one test produces
a huge diff on this file.

.gitattributes

@@ -1,2 +1,4 @@
-# Declare files that will always have LF line endings on checkout.
+# Declare files will always have LF line endings on checkout.
 configure eol=lf
+
+tests/dune.inc linguist-generated