Add more tests for name type checks in setxattr, getxattr, fsetxattr, etc

[humphd]

We have a bunch of error paths not getting hit in our tests for fs.setxattr and fs.fsetxattr related to checking on the type of the name argument: https://codecov.io/gh/filerjs/filer/src/master/src/filesystem/implementation.js#L1404.

We need to add checks for passing a name that is the empty string, passing null/undefined/Object/etc non-strings for name to setxattr and fsetxattr, and passing invalid flags.