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/unordered/rehash_tests.cpp

@@ -22,7 +22,7 @@ test::seed_t initialize_seed(2974);
 template <class X>
 bool postcondition(X const& x, BOOST_DEDUCED_TYPENAME X::size_type n)
 {
-    return static_cast<double>(x.bucket_count()) >
+    return static_cast<double>(x.bucket_count()) >=
         static_cast<double>(x.size()) / x.max_load_factor() &&
         x.bucket_count() >= n;
 }
@@ -149,9 +149,6 @@ void reserve_test1(X*, test::random_generator generator)
             X x;
             x.max_load_factor(random_mlf ?
                 static_cast<float>(std::rand() % 1000) / 500.0f + 0.5f : 1.0f);
-            // For the current standard this should reserve i+1, I've
-            // submitted a defect report and will assume it's a defect
-            // for now.
             x.reserve(test::has_unique_keys<X>::value ? i : v.size());
 
             // Insert an element before the range insert, otherwise there are