Gate enum tests that depend on Ref.Emit (#50093)

In #49925 I blindly followed the existing pattern but the existing pattern was wrong because with that we get failing tests:

```
System.InvalidOperationException : No data found for System.Tests.EnumTests.IsDefined_UnsupportedEnumType_ThrowsInvalidOperationException
```

src/libraries/System.Runtime/tests/System/EnumTests.cs

@@ -1979,23 +1979,20 @@ public static void Format_Invalid()
 
         public static IEnumerable<object[]> UnsupportedEnumType_TestData()
         {
-            if (PlatformDetection.IsReflectionEmitSupported)
-            {
-                yield return new object[] { s_floatEnumType, 1.0f };
-                yield return new object[] { s_doubleEnumType, 1.0 };
-                yield return new object[] { s_intPtrEnumType, (IntPtr)1 };
-                yield return new object[] { s_uintPtrEnumType, (UIntPtr)1 };
-            }
+            yield return new object[] { s_floatEnumType, 1.0f };
+            yield return new object[] { s_doubleEnumType, 1.0 };
+            yield return new object[] { s_intPtrEnumType, (IntPtr)1 };
+            yield return new object[] { s_uintPtrEnumType, (UIntPtr)1 };
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsReflectionEmitSupported))]
         [MemberData(nameof(UnsupportedEnumType_TestData))]
         public static void GetName_Unsupported_ThrowsArgumentException(Type enumType, object value)
         {
             AssertExtensions.Throws<ArgumentException>("value", () => Enum.GetName(enumType, value));
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsReflectionEmitSupported))]
         [MemberData(nameof(UnsupportedEnumType_TestData))]
         public static void IsDefined_UnsupportedEnumType_ThrowsInvalidOperationException(Type enumType, object value)
         {
@@ -2006,16 +2003,13 @@ public static void IsDefined_UnsupportedEnumType_ThrowsInvalidOperationException
 
         public static IEnumerable<object[]> UnsupportedEnum_TestData()
         {
-            if (PlatformDetection.IsReflectionEmitSupported)
-            {
-                yield return new object[] { Enum.ToObject(s_floatEnumType, 1) };
-                yield return new object[] { Enum.ToObject(s_doubleEnumType, 2) };
-                yield return new object[] { Enum.ToObject(s_intPtrEnumType, 1) };
-                yield return new object[] { Enum.ToObject(s_uintPtrEnumType, 2) };
-            }
+            yield return new object[] { Enum.ToObject(s_floatEnumType, 1) };
+            yield return new object[] { Enum.ToObject(s_doubleEnumType, 2) };
+            yield return new object[] { Enum.ToObject(s_intPtrEnumType, 1) };
+            yield return new object[] { Enum.ToObject(s_uintPtrEnumType, 2) };
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsReflectionEmitSupported))]
         [MemberData(nameof(UnsupportedEnum_TestData))]
         public static void ToString_UnsupportedEnumType_ThrowsArgumentException(Enum e)
         {
@@ -2024,7 +2018,7 @@ public static void ToString_UnsupportedEnumType_ThrowsArgumentException(Enum e)
             Assert.True(formatXExceptionName == nameof(InvalidOperationException) || formatXExceptionName == "ContractException");
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsReflectionEmitSupported))]
         [MemberData(nameof(UnsupportedEnumType_TestData))]
         public static void Format_UnsupportedEnumType_ThrowsArgumentException(Type enumType, object value)
         {