Fixed #15020

[KN4CK3R]

Addition for #15020.

assert.Equals itself prints both values so the previous fix was only half the work.

[KN4CK3R]

Alternative implementation could be

if diffStr != actual {
  assert.Fail(t, "Unexpected change in the diff string: expected: %s but was actually: %s", diffStr[:expectedMaxLen], actual[:actualMaxLen])
}

if the Should be true message is unwanted.