enable unicode tests on OSX (#1728)

src/libraries/System.IO.Compression.ZipFile/tests/ZipFile.Extract.cs

@@ -34,7 +34,7 @@ public void ExtractToDirectoryNull()
             AssertExtensions.Throws<ArgumentNullException>("sourceArchiveFileName", () => ZipFile.ExtractToDirectory(null, GetTestFilePath()));
         }
 
-        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotMacOsHighSierraOrHigher))]
+        [Fact]
         public void ExtractToDirectoryUnicode()
         {
             string zipFileName = zfile("unicode.zip");

src/libraries/System.IO.Compression.ZipFile/tests/ZipFileExtensions.ZipArchive.Extract.cs

@@ -22,7 +22,7 @@ public void ExtractToDirectoryExtension()
             }
         }
 
-        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotMacOsHighSierraOrHigher))]
+        [Fact]
         public void ExtractToDirectoryExtension_Unicode()
         {
             using (ZipArchive archive = ZipFile.OpenRead(zfile("unicode.zip")))