Change the CheckCtx constructor to take a Name enum

This prepares to eliminate some reliance on string matching but does not
yet make those changes.

library/compiler-builtins/libm/crates/libm-test/benches/random.rs

@@ -50,7 +50,7 @@ where
     let name = Op::NAME_STR;
 
     let ulp = libm_test::musl_allowed_ulp(name);
-    let ctx = CheckCtx::new(ulp, name, CheckBasis::Musl);
+    let ctx = CheckCtx::new(ulp, Op::NAME, CheckBasis::Musl);
     let benchvec: Vec<_> =
         random::get_test_cases::<Op::RustArgs>(&ctx).take(BENCH_ITER_ITEMS).collect();