Also test coretests when running ./x.py test core

It is reasonable to expect that ./x.py test core is enough to run tests
when you are working on core. In addition it seems like CI for wasm32 at
least doesn't run coretests currently, which this commit fixes.

Author: bjorn3

src/bootstrap/src/core/build_steps/test.rs

@@ -2785,14 +2785,15 @@ impl Step for Crate {
             _ => panic!("can only test libraries"),
         };
 
-        run_cargo_test(
-            cargo,
-            &[],
-            &self.crates,
-            &*crate_description(&self.crates),
-            target,
-            builder,
-        );
+        let mut crates = self.crates.clone();
+        // The core crate can't directly be tested. We could silently
+        // ignore it, but adding it's own test crate is less confusing
+        // for users. We still keep core itself for doctests.
+        if crates.iter().any(|crate_| crate_ == "core") {
+            crates.push("coretests".to_owned());
+        }
+
+        run_cargo_test(cargo, &[], &crates, &*crate_description(&self.crates), target, builder);
     }
 }