use ConditionalFact instead of SkipOnTargetFramework

Author: krwq

src/System.Security.Cryptography.Xml/tests/Samples/SigningVerifyingX509Cert.cs

@@ -19,6 +19,8 @@ public class SigningVerifyingX509Cert
 <example>
 <test>some text node</test>
 </example>";
+        
+        private static bool SupportsDecryptValue => !PlatformDetection.IsFullFramework || PlatformDetection.IsNetfx462OrNewer();
 
         private static void SignXml(XmlDocument doc, AsymmetricAlgorithm key)
         {
@@ -59,14 +61,8 @@ private static bool VerifyXml(string signedXmlText, X509Certificate2 certificate
             return signedXml.CheckSignature(certificate, verifySignatureOnly: true);
         }
 
-        [Fact]
-        [SkipOnTargetFramework(
-            TargetFrameworkMonikers.Net45 |
-            TargetFrameworkMonikers.Net451 |
-            TargetFrameworkMonikers.Net452 |
-            TargetFrameworkMonikers.Net46 |
-            TargetFrameworkMonikers.Net461,
-            "https://github.com/dotnet/corefx/issues/19270")]
+        // https://github.com/dotnet/corefx/issues/19270
+        [ConditionalFact(nameof(SupportsDecryptValue))]
         public void SignedXmlHasCertificateVerifiableSignature()
         {
             using (X509Certificate2 x509cert = TestHelpers.GetSampleX509Certificate())