Use OS specific default backend in integration tests

[webmaster128]

Unfortunately Singlepass does not work on Windows. In order to prevent frustration for contract developers, we use Cranelift by default for contract development (integration tests).

Once rust-lang/cargo#7914 is available, we can do:

[target.'cfg(not(target_os = "windows"))'.features]
default = []

[target.'cfg(target_os = "windows")'.features]
default = ["cranelift"]