Arrow function tests: remove lots of code duplication
Arrow function tests: remove lots of code duplication

* Simplify the arrow function testing by using a helper function to check the token positions.
* Fix incorrect parameter order of the `$expected` vs `$result` parameters in `assertSame()`.
    While when the test passes, this makes no difference, it does make a difference in the error output when a test fails.

squizlabs/PHP_CodeSniffer,squizlabs/PHP_CodeSniffer,jrfnl/PHP_CodeSniffer,stronk7/PHP_CodeSniffer,stronk7/PHP_CodeSniffer,jrfnl/PHP_CodeSniffer,squizlabs/PHP_CodeSniffer,jrfnl/PHP_CodeSniffer,stronk7/PHP_CodeSniffer