Rollup merge of #85287 - eggyal:expose-test-concurrent, r=m-ou-se

Expose `Concurrent` (private type in public i'face)

#53410 introduced experimental support for custom test frameworks.

Such frameworks may wish to build upon `library/test` by calling into its publicly exposed API (which I entirely understand is wholly unstable).  However, any that wish to call `test::run_test` cannot currently do so because `test::options::Concurrent` (the type of its `concurrent` parameter) is not publicly exposed.

library/test/src/lib.rs

@@ -49,7 +49,7 @@ pub mod test {
         cli::{parse_opts, TestOpts},
         filter_tests,
         helpers::metrics::{Metric, MetricMap},
-        options::{Options, RunIgnored, RunStrategy, ShouldPanic},
+        options::{Concurrent, Options, RunIgnored, RunStrategy, ShouldPanic},
         run_test, test_main, test_main_static,
         test_result::{TestResult, TrFailed, TrFailedMsg, TrIgnored, TrOk},
         time::{TestExecTime, TestTimeOptions},