Let miri decide the flags to use for the test suite

src/bootstrap/test.rs

@@ -572,8 +572,6 @@ impl Step for Miri {
                 return;
             }
 
-            // # Run `cargo test` with `-Zmir-opt-level=4`.
-            cargo.env("MIRIFLAGS", "-O -Zmir-opt-level=4");
             if !try_run(builder, &mut cargo) {
                 return;
             }