Rollup merge of #131729 - Urgau:check-cfg-test-userspace, r=petrochenkov

Make the `test` cfg a userspace check-cfg

This PR implements MCP https://github.com/rust-lang/compiler-team/issues/785, which makes the `test` cfg a "userspace" check-cfg, i.e. no longer included in the well known cfg list.

Things to do:

- [x] Accept the MCP (https://github.com/rust-lang/compiler-team/issues/785#issuecomment-2424121886)
- [x] Mark `test` in Cargo (https://github.com/rust-lang/cargo/pull/14963)

`@rustbot` labels +S-waiting-on-MCP +F-check_cfg
r? `@petrochenkov`

compiler/rustc_session/src/config/cfg.rs

@@ -332,6 +332,11 @@ impl CheckCfg {
         // Note that symbols inserted conditionally in `default_configuration`
         // are inserted unconditionally here.
         //
+        // One exception is the `test` cfg which is consider to be a "user-space"
+        // cfg, despite being also set by in `default_configuration` above.
+        // It allows the build system to "deny" using the config by not marking it
+        // as expected (e.g. `lib.test = false` for Cargo).
+        //
         // When adding a new config here you should also update
         // `tests/ui/check-cfg/well-known-values.rs` (in order to test the
         // expected values of the new config) and bless the all directory.
@@ -453,8 +458,6 @@ impl CheckCfg {
 
         ins!(sym::target_thread_local, no_values);
 
-        ins!(sym::test, no_values);
-
         ins!(sym::ub_checks, no_values);
 
         ins!(sym::unix, no_values);