CWD for README.md commands

[jayvdb]

The sequence in Electron100/butane@c5761cc#diff-b70d3a47c15879d308451b54821682f1d63518db732881b434c4110d9ca7a767 assumes that the user is in the directory containing the .md file. I think that is a reasonable default for trycmd. I don't see any set_current_dir in this project.

It would also be useful to add something to https://docs.rs/trycmd/latest/trycmd/struct.TestCases.html to allow setting the CWD for that set of cases.

[epage]

Current behavior is

• .toml files can set CWD explicitly
• Default CWD to path.with_ext(".in") if it exists
• Otherwise, default to parent process CWD which is the manifest dir

See also https://github.com/assert-rs/trycmd/blob/main/src/schema.rs#L85

Most of those cases listed came out specific use cases but the CWD never came up in a case without a .in, so I hadn't really considered.

Since we treat everything else relative to path.parent(), I think it makes sense to make this change.

This will be a breaking change though