Throw the same exception in DEBUG and non-DEBUG tests

AssertFailureException is something I want to make internal to the
TraceListener, so let's just simplify this code.

src/Compilers/Core/CodeAnalysisTest/RealParserTests.cs

@@ -403,17 +403,10 @@ static void CheckOneDouble(string s, double expected)
             if (!RealParser.TryParseDouble(s, out actual)) actual = 1.0 / 0.0;
             if (!actual.Equals(expected))
             {
-#if DEBUG
-                throw new AssertFailureException($@"
-Error for double input ""{s}""
-   expected {InvariantToString(expected)}
-   actual {InvariantToString(actual)}");
-#else
                 throw new Exception($@"
 Error for double input ""{s}""
    expected {InvariantToString(expected)}
    actual {InvariantToString(actual)}");
-#endif
             }
         }
 
@@ -585,15 +578,9 @@ static void CheckOneFloat(string s, float expected)
             if (!RealParser.TryParseFloat(s, out actual)) actual = 1.0f / 0.0f;
             if (!actual.Equals(expected))
             {
-#if DEBUG
-                throw new AssertFailureException($@"Error for float input ""{s}""
-   expected {InvariantToString(expected)}
-   actual {InvariantToString(actual)}");
-#else
                 throw new Exception($@"Error for float input ""{s}""
    expected {InvariantToString(expected)}
    actual {InvariantToString(actual)}");
-#endif
             }
         }