Function prepare_cargo_test

fn prepare_cargo_test(
    cargo: impl Into<BootstrapCommand>,
    libtest_args: &[&str],
    crates: &[String],
    primary_crate: &str,
    compiler: Compiler,
    target: TargetSelection,
    builder: &Builder<'_>,
) -> BootstrapCommand

Given a cargo test subcommand, pass it the appropriate test flags given a builder.