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

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:

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

_Originally posted by @ermshiperete in https://github.com/keymanapp/keyman/pull/15923#discussion_r3207657950_