Change default ULP to use enum matching

Migrate from string to enum matching and tie this to `CheckCtx::new`, so
no tests need to explicitly set ULP.

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

@@ -49,8 +49,7 @@ where
 {
     let name = Op::NAME;
 
-    let ulp = libm_test::musl_allowed_ulp(name);
-    let ctx = CheckCtx::new(ulp, Op::IDENTIFIER, CheckBasis::Musl);
+    let ctx = CheckCtx::new(Op::IDENTIFIER, CheckBasis::Musl);
     let benchvec: Vec<_> =
         random::get_test_cases::<Op::RustArgs>(&ctx).take(BENCH_ITER_ITEMS).collect();