std_detect testing improvements

* Fix riscv testing. Previously the mod tests; would be looking for
  src/detect/os/tests.rs.
* Replace a test with an unnamed const item. It is testing that no
  warnings are emitted. It doesn't contain any checks that need to run
  at runtime. Replacing the test allows removing the tidy:skip directive
  for test locations.

library/std_detect/src/detect/macros.rs

@@ -131,14 +131,13 @@ macro_rules! features {
             };
         }
 
-        #[test] //tidy:skip
         #[deny(unexpected_cfgs)]
         #[deny(unfulfilled_lint_expectations)]
-        fn unexpected_cfgs() {
+        const _: () = {
             $(
                 check_cfg_feature!($feature, $feature_lit $(, without cfg check: $feature_cfg_check)? $(: $($target_feature_lit),*)?);
             )*
-        }
+        };
 
         /// Each variant denotes a position in a bitset for a particular feature.
         ///

library/std_detect/src/detect/os/riscv.rs

@@ -135,4 +135,5 @@ pub(crate) fn imply_features(mut value: cache::Initializer) -> cache::Initialize
 }
 
 #[cfg(test)]
+#[path = "riscv/tests.rs"]
 mod tests;