Fix tests on cs-CZ culture (#21171)

Merged on behalf of @svick 
Thanks for the contribution!

src/Compilers/Core/CodeAnalysisTest/Text/StringTextDecodingTests.cs

@@ -133,7 +133,7 @@ public void CheckSum_SHA256()
             Assert.Equal("f1945cd6 c19e56b3 c1c78943 ef5ec181 16907a4c a1efc40a 57d48ab1 db7adfc5", StringTextTest.ChecksumToHexQuads(checksum));
         }
 
-        [ConditionalFact(typeof(IsEnglishLocal))]
+        [ConditionalFact(typeof(HasEnglishDefaultEncoding))]
         [WorkItem(5663, "https://github.com/dotnet/roslyn/issues/5663")]
         public void Decode_NonUtf8()
         {

src/Test/Utilities/Portable/Assert/ConditionalFactAttribute.cs

@@ -47,6 +47,13 @@ public class HasShiftJisDefaultEncoding : ExecutionCondition
         public override string SkipReason => "OS default codepage is not Shift-JIS (932).";
     }
 
+    public class HasEnglishDefaultEncoding : ExecutionCondition
+    {
+        public override bool ShouldSkip => Encoding.GetEncoding(0)?.CodePage != 1252;
+
+        public override string SkipReason => "OS default codepage is not Windows-1252.";
+    }
+
     public class IsEnglishLocal : ExecutionCondition
     {
         public override bool ShouldSkip =>

src/Test/Utilities/Portable/Compilation/OperationTreeVerifier.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Globalization;
 using System.Linq;
 using System.Text;
 using System.Text.RegularExpressions;
@@ -163,13 +164,28 @@ private void LogConstant(Optional<object> constant, string header = "Constant")
             }
         }
 
-        private void LogConstant(object constant, string header = "Constant")
+        private static string ConstantToString(object constant, bool quoteString = true)
         {
-            var valueStr = constant != null ? constant.ToString() : "null";
-            if (constant is string)
+            switch (constant)
             {
-                valueStr = @"""" + valueStr + @"""";
+                case null:
+                    return "null";
+                case string s:
+                    if (quoteString)
+                    {
+                        return @"""" + s + @"""";
+                    }
+                    return s;
+                case IFormattable formattable:
+                    return formattable.ToString(null, CultureInfo.InvariantCulture);
+                default:
+                    return constant.ToString();
             }
+        }
+
+        private void LogConstant(object constant, string header = "Constant")
+        {
+            string valueStr = ConstantToString(constant);
 
             LogString($"{header}: {valueStr}");
         }
@@ -845,8 +861,7 @@ public override void VisitLiteralExpression(ILiteralExpression operation)
         {
             LogString(nameof(ILiteralExpression));
 
-            object value;
-            if (operation.ConstantValue.HasValue && ((value = operation.ConstantValue.Value) == null ? "null" : value.ToString()) == operation.Text)
+            if (operation.ConstantValue.HasValue && ConstantToString(operation.ConstantValue.Value, quoteString: false) == operation.Text)
             {
                 LogString($" (Text: {operation.Text})");
             }