Fix miri by ensuring that core and alloc don't get tested directly

cargo test -p core -p alloc overrides the test=false in Cargo.toml so we
have to avoid passing -p core and -p alloc to cargo test.

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

@@ -2786,12 +2786,15 @@ impl Step for Crate {
         };
 
         let mut crates = self.crates.clone();
-        // The core crate can't directly be tested. We could silently
-        // ignore it, but replacing it with it's own test crate is less
-        // confusing for users.
+        // The core and alloc crates can't directly be tested. We could
+        // silently ignore them, but replacing them with their test crate
+        // is less confusing for users.
         if crates.iter().any(|crate_| crate_ == "core") {
             crates.push("coretests".to_owned());
         }
+        if crates.iter().any(|crate_| crate_ == "alloc") {
+            crates.push("alloctests".to_owned());
+        }
 
         run_cargo_test(cargo, &[], &crates, &*crate_description(&self.crates), target, builder);
     }