tests/test-diff-newlines.out

tests: fix bashism to load helpers.sh

`source` isn't available at POSIX sh.

adding a
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a	Thu Jan 01 00:00:01 1970 +0000
+++ b/a	Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@
 confuse str.splitlines
 embedded
newline
+clean diff