Unit Tests for the __return functions in functions.php

[pbearne]

Just a unit test for some functions in the functions.php part of my ongoing effort to add missing tests

[pbearne]

Unittest

[johnbillion]

test__return_zero() and test__return_empty_string() need to use assertSame() instead of assertEquals() otherwise they use loose type comparison, which makes the tests unreliable.

test__return_true() and test__return_false() shouldn't cast to boolean otherwise they too are unreliable.

[mukesh27]

Updated Patch as suggested by @johnbillion

[pbearne]

Updated the empty array test to use assertSame as well

[pento]

In 44658:

Tests: Add tests for the __return*() functions.

Props pbearne, mukesh27.
Fixes #46043.