Update tests for standard changes involving bucket count.

It seems my defect report was accepted at some point, and they tweaked
the requirements involving bucket counts. This also makes it possible to
have a bucket count of 0, which I think wasn't allowed in the past. I
don't think I'll change this implementation to do so, but I'd like to be
able to run these tests against standard implementations, so I'm
starting to take that into account.

I believe these changes were made after the C++14 standard, but I've
always been tracking the draft standards, so that doesn't really matter.

test/helpers/invariants.hpp

@@ -91,7 +91,7 @@ namespace test
 
         // Check the load factor.
 
-        float load_factor =
+        float load_factor = size == 0 ? 0 :
             static_cast<float>(size) / static_cast<float>(x1.bucket_count());
         using namespace std;
         if(fabs(x1.load_factor() - load_factor) > x1.load_factor() / 64)