Add Windows-specific comments to testing guide

[steveklabnik]

Fixes #27936

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[retep998]

Don't forget to mention that %ERRORLEVEL% is only when you're inside cmd. Inside an msys environment you use the same thing like you'd use on linux and OS X. In powershell you use either $LASTEXITCODE or $? depending upon whether you want an exist code or just a bool respectively.

[steveklabnik]

@retep998 updated, what do you think?

[retep998]

It's better.

[alexcrichton]

@bors: r+ af58e5c