[9.x] Fix schedule:list crash when call() is given class-string

[odinns]

When creating schedule of the form:

$this->schedule->call(FooCall::class);
$this->schedule->call([FooCall::class, 'fooFunction']);

Then the ScheduleListCommand::getClosureLocation() method would crash with:

TypeError

Cannot use "::class" on value of type string

This PR fixes it by adding checks for string $callback and returning that string instead of adding ::class to it.

Tests for both single string and array callable syntax included.

[odinns]

Thank you to @lukeraymonddowning and @olivernybroe for help with the solution! 😊👍