Substitute vars in tests

[9999years]

1. Substitute the path variables used in the tests. This makes the tests a lot easier to read, because I don't need to do the substitution in my head.
2. Add some doctests for relative paths, because it wasn't clear from reading the docs what the behavior would be.

[Manishearth]

Oops, should have waited for all CI to pass

---- src\lib.rs - diff_paths (line 18) stdout ----
Test executable failed (exit code: 101).

stderr:
thread 'main' panicked at src\lib.rs:16:1:
assertion `left == right` failed
  left: Some("..\\..\\..\\baz")
 right: None
stack backtrace:
   0:     0x7ff6b7cbdc2a - std::backtrace_rs::backtrace::dbghelp64::trace
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\..\..\backtrace\src\backtrace\dbghelp64.rs:99
   1:     0x7ff6b7cbdc2a - std::backtrace_rs::backtrace::trace_unsynchronized
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\..\..\backtrace\src\backtrace\mod.rs:66
   2:     0x7ff6b7cbdc2a - std::sys_common::backtrace::_print_fmt
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\sys_common\backtrace.rs:68
   3:     0x7ff6b7cbdc2a - std::sys_common::backtrace::_print::impl$0::fmt
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\sys_common\backtrace.rs:44
   4:     0x7ff6b7ccb8fb - core::fmt::rt::Argument::fmt
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\fmt\rt.rs:142
   5:     0x7ff6b7ccb8fb - core::fmt::write
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\fmt\mod.rs:1153
   6:     0x7ff6b7cbaf91 - std::io::Write::write_fmt<std::sys::pal::windows::stdio::Stderr>
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\io\mod.rs:1843
   7:     0x7ff6b7cbda16 - std::sys_common::backtrace::print
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\sys_common\backtrace.rs:[34](https://github.com/Manishearth/pathdiff/actions/runs/9490348669/job/26155430777#step:4:35)
   8:     0x7ff6b7cbfaaf - std::panicking::default_hook::closure$1
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\panicking.rs:272

[Manishearth]

Opened #12 to undo that test. If you can investigate what's going on there we can add it again either as a cfg'd test or as a regular test with some behavior fixed.