test: Unsafe usage of `env::set_var`

[olix0r]

In the proxy's test initialization code, we use std::env::set_var to configure a log format:

pub fn trace_subscriber(default: impl ToString) -> (Dispatch, Handle) {
    let log_level = env::var("LINKERD2_PROXY_LOG")
        .or_else(|_| env::var("RUST_LOG"))
        .unwrap_or_else(|_| default.to_string());
    let log_format = env::var("LINKERD2_PROXY_LOG_FORMAT").unwrap_or_else(|_| "PLAIN".to_string());
    env::set_var("LINKERD2_PROXY_LOG_FORMAT", &log_format);
    // This may fail, since the global log compat layer may have been
    // initialized by another test.
    let _ = init_log_compat();
    Settings::for_test(log_level, log_format).build()
}

My understanding is that this is unsafe, per rust-lang/rust#90308, because our tests run in multiple threads. It would be great if we could avoid environment mutation in our codebase.

Can we remove this? Do we actually care about JSON formatting in our tests?

Are there tools we could use to, for instance, make clippy fail on new uses of set_var/remove_var?

[hawkw]

In this case, we can just remove the set_var calls. I never understood why they were there to begin with --- I think this is an artifact of back before we were able to use the same logging configuration for the test driver thread and the test proxy?