Fix the scarb-test-support integration test hack for generating the binary

[MrDenkoV]

Test ctrl_c_kills_everyone in tests/subcommand.rs uses cargo_bin("scarb-test-support"), which depends on existence of "scarb-test-support" binary in the target dir.
To enforce cargo test to build "scarb-test-support" binary we created empty integration tests for scarb-test-support.
It is a hack, fix it once rust-lang/cargo#9096 is finished and released.