Force Unix-style line-endings for some helper files in staticfiles_tests.

This fixes eight failures in the cached storage tests under Windows.

Refs #18920 and 978d4476cf.

.gitattributes

@@ -1,2 +1,5 @@
 # Normalize line endings to avoid spurious failures in the core test suite on Windows.
-*html text eol=lf
+*html text eol=lf
+*css text eol=lf
+tests/staticfiles_tests/apps/test/static/test/*txt text eol=lf
+tests/staticfiles_tests/project/documents/test/*txt text eol=lf