pdl: simplify ‘assert_eq_with_diff’

Before we would print the full “left” and “right” texts. However, with
snapshot testing (coming in https://r.android.com/c/2098984/3) this is
not really necessary any longer.

Bug: 228307941
Test: atest pdl_inline_tests
Change-Id: I8b1c348ecef64ff5ab677e2040eb645d9af4e236