tests:os/fs:Add check if changing directory worked

* The former test only cheked if `os_chdir` did return 0 to signal
  that everything went write. This adds a check to see if the
  current directory is actually the one we changed to.
tests:os/fs: Test os_get_file_size on not-empty file

* The former test checked only if the file size of an empty file is 0, this
  changes the test to an non empty file to be sure the size variable doesn't
  just have the standard value of 0.
tests:os/fs fix typo in test description

* Fix a typo in one of `os_chdir` test descriptions.