bzr: Changed bzr tests for windows compatibility

This PR patches the bzr package for compatibility on Windows, taking into account the different home location.
Use of underscores is not idiomatic for Go. Use lowercase, camelcased names, please (http://golang.org/doc/effective_go.html#names).

This should also be one constant called bzrHome. Define it separately in windows-specific (*_windows.go) and unix-specific (*_unix.go) source files. Then you won't have to case-switch on GOOS below.

See http://golang.org/pkg/go/build/, section "Build Constraints" for how to set up os-specific source files.