* Rename Two Test Files
When I run unit tests only for Reader/Xlsx, phpunit is issuing a deprecation message because the names of 2 files have an extra dot in them and thus don't match the class name in the file. I do not see these warnings when I run the entire test suite.
* Remove Phpstan Annotations
It was a bit difficult to handle a cast from mixed to string.
* Fix Same Phpstan Problem in One Other Test
This is the only other test case that tries to cast mixed to string.
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