* fs/test.zig: use arena allocator more consistently

* fs/test.zig: remove unnecessary type information

Zig can (now?) implicitly cast a `&.{ "foo"}` when passed to
`fs.path.join()`, so the `[_][]const u8` is unnecessary.

* fs/test.zig: Use fs.path.join() for longer paths

Replace long path constructions (that use several "++ path_sep ++")
with a single call to `fs.path.join`.  Seems more readable to me.

* fs/test.zig: fmt