libtest should set RUST_TEST_THREADS environment variable

[nbaksalyar]

We can configure the test runner concurrency by using either the --test-threads parameter, or by setting the environment variable RUST_TEST_THREADS:

rust/src/libtest/lib.rs

Line 245 in efc02b0

let concurrency = opts.test_threads.unwrap_or_else(get_concurrency);

As some tests might need to read the current concurrency configuration, the only way to do this currently is by reading the said environment variable. However, if we use --test-threads, this information is not passed further to tests. I think this behaviour is a bit inconsistent and the env var should be always set to the actual value, e.g.:

let concurrency = opts.test_threads.unwrap_or_else(get_concurrency);
env::set_var("RUST_TEST_THREADS", concurrency.to_string());

[bjorn3]

I think the name would be ambiguous when using an external process to run the tests (used for -Cpanic=abort only targets) It could refer to the total amount of concurrenr threads or the total amount of threads in the current process (1)