Add test helper macros. (#465)

- Refactor the existing SNMALLOC_ASSERT and SNMALLOC_CHECK.  These now
   use the FatalErrorBuilder to format the output if a format string is
   provided.
 - Extend the FatalErrorBuilder to print decimal integers for signed
   values.
 - Rename FatalErrorBuilder to MessageBuilder.
 - Rewrite the macros used in the jemalloc tests to use
   FatalErrorBuilder and move them into a header.
 - Refactor some of the tests to use the new macros.

src/pal/pal.h

@@ -161,10 +161,10 @@ namespace snmalloc
    *
    *  These types should be sufficient for allocator-related error messages.
    */
-  template<size_t BufferSize = 1024, typename... Args>
+  template<size_t BufferSize, typename... Args>
   [[noreturn]] inline void report_fatal_error(Args... args)
   {
-    FatalErrorBuilder<BufferSize> msg{std::forward<Args>(args)...};
+    MessageBuilder<BufferSize> msg{std::forward<Args>(args)...};
     Pal::error(msg.get_message());
   }