Use exception in CappedStringWriter

The use of `Assert.True` is problematic here as it's throwing an xunit
defined exception. In this case `TrueException`. This is problematic
because it's not serializable yet it is callable from within our
isolated test `AppDomain` instances.

In this case we should just be throwing an exception directly here.

src/Test/Utilities/Portable/FX/CappedStringWriter.cs

@@ -33,7 +33,7 @@ public CappedStringWriter(int expectedLength)
 
         private void CapReached()
         {
-            Assert.True(false, "Test produced more output than expected (" + _expectedLength + " characters). Is it in an infinite loop? Output so far:\r\n" + GetStringBuilder());
+            throw new Exception($"Test produced more output than expected ({_expectedLength} characters). Is it in an infinite loop? Output so far:\r\n{GetStringBuilder()}");
         }
 
         public override void Write(char value)