This makes the tests more hermetic, even though I don't think the
default values (taken from `whoami`) can break any tests (then we
would have already seen them break). Now we just need to make the
operation log's timestamps predictable and then we can start using
operation IDs in snapshot tests.
There are no "non-normal" files, so "normal" is not needed. We have
symlinks and conflicts, but they are not files, so I think just "file"
is unambiguous.
I left `testutils::write_normal_file()` because there it's used to
mean "not executable file" (there's also a `write_executable_file()`).
I left `working_copy::FileType::Normal` since renaming `Normal` there
to `File` would also suggest we should rename `FileType`, and I don't
know what would be a better name for that type.
The `testutils` module should ideally not be part of the library
dependencies. Since they're used by the integration tests (and the CLI
tests), we need to move them to a separate crate to achieve that.