Miri: disable a slow test

Author: RalfJung

src/libcore/tests/num/dec2flt/mod.rs

@@ -31,6 +31,7 @@ fn ordinary() {
     test_literal!(0.1);
     test_literal!(12345.);
     test_literal!(0.9999999);
+    #[cfg(not(miri))] // Miri is too slow
     test_literal!(2.2250738585072014e-308);
 }
 
@@ -77,6 +78,7 @@ fn zero() {
     test_literal!(0.0);
     test_literal!(1e-325);
     test_literal!(1e-326);
+    #[cfg(not(miri))] // Miri is too slow
     test_literal!(1e-500);
 }