[5.3] Consolidate tests for first() and last() Collection methods

[vlakoff]

Disclaimer: this one is a nitpick, but tidying the tests is not a luxury :)

Current situation, note the gap between lines 45 and 960:

Search "test(First|Last)" (8 hits)
    Line 10:     public function testFirstReturnsFirstItemInCollection()
    Line 16:     public function testLastReturnsLastItemInCollection()
    Line 23:     public function testLastWithCallback()
    Line 36:     public function testLastWithCallbackAndDefault()
    Line 45:     public function testLastWithDefaultAndWithoutCallback()
    Line 960:    public function testFirstWithCallback()
    Line 969:    public function testFirstWithCallbackAndDefault()
    Line 978:    public function testFirstWithDefaultAndWithoutCallback()

This gap was introduced in https://github.com/laravel/framework/pull/2858/files#diff-f565fa2deb7f67254a3a052a4f1d214b.