Use existing Interop.IsWindows instead of inventing a new detection logic.

Author: bartonjs

src/System.Security.Cryptography.X509Certificates/tests/System.Security.Cryptography.X509Certificates.Tests.csproj

@@ -31,6 +31,9 @@
     <Compile Include="PropsTests.cs" />
     <Compile Include="PublicKeyTests.cs" />
     <Compile Include="TestData.cs" />
+    <Compile Include="$(CommonPath)\Interop\Interop.PlatformDetection.cs">
+      <Link>Common\Interop\Interop.PlatformDetection.cs</Link>
+    </Compile>
   </ItemGroup>
   <ItemGroup>
     <None Include="project.json" />

src/System.Security.Cryptography.X509Certificates/tests/TestData.cs

@@ -373,7 +373,7 @@ internal static class TestData
 
         internal static string NormalizeX500String(string expected)
         {
-            if (StringComparer.Ordinal.Equals("Windows_NT", Environment.GetEnvironmentVariable("OS")))
+            if (Interop.IsWindows)
             {
                 return expected;
             }