[10.x] Improve tests for Arr::first and Arr::last

[tamiroh]

Since the tests for Arr::last were not sufficient compared to Arr::first, I added tests to testLast() that were almost identical to those included in testFirst().

In addition, there was a test for Arr::last that was not contained in Arr::first, thus I added it to testFirst().

As a result, the tests for Arr::first and Arr::last are now nearly equivalent, which is reasonable since Arr::first and Arr::last are expected to be symmetrical in their behavior.