Add tests for ~tilde expansion:
- test that "~" is expanded (no more "~" in output)
- ensure that "1~1" is not expanded to "1/home/user1" as it was
before
Fixes#972
Note: the first test does not check the literal expansion because
the path on Windows is expanded as a Linux path, but the correct
expansion may come for free once `shellexpand` will use the `dirs`
crate too (https://github.com/netvl/shellexpand/issues/3).