When seeking in a file to a location before the beginning of the file,
the operation seems to succeed. While if i try to do the same with a
"real" file then the seek command will return an error.

So i changed the seek method to return false if the offset is less
than 0. This seems to solve my problem and behaves in the same
way as seeking in a real file. However I do not know if there are
situations where seeking before the beginning of the file could be
a valid use case. testing with fseek on unix fs always returns -1
when seeking past the beginning of the file. Added five test
assertions to validate this behavior.

Implemented pull request feedback:

- Refactor seek so internal offset is not modified when the seek fails.

- Moved "seek before beginning" assertions into their own test methods.