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/exception/insert_exception_tests.cpp

@@ -130,6 +130,7 @@ struct insert_test_rehash1 : public insert_test_base<T>
 
         T x;
         x.max_load_factor(0.25);
+        // TODO: This doesn't really work is bucket_count is 0
         size_type bucket_count = x.bucket_count();
         size_type initial_elements = static_cast<size_type>(
             ceil(bucket_count * (double) x.max_load_factor()) - 1);