PDL: Fix the order of arguments to ‘assert_eq_with_diff’

We now show the unified diff relative to the known-good code. That is,
if we generate a line too much, this like shows up as an added line.
Before it would show up as a deleted line, which seems wrong.

Test: atest pdl_inline_tests
Change-Id: I9ecb24c4f79b65221131adfe0e44db4529a6a478