chore(core): use cleaner gtest patterns for error diagnostics

[mcdurdin]

Instead of having the if (dst_nfd != expect_nfd) block above that conditionally outputs the values, we could write a function that does that and then change the assert to:

  ASSERT_EQ(dst_nfd, expect_nfd) << OutputValues(dst_nfd, expect_nfd);

Originally posted by @ermshiperete in #15923 (comment)