Remove version(assert) from TimSort

If `-debug` is passed to the compiler, `version(assert)` is true.
TimSort has a `version(assert)` that checks some invariants, and uses
std.format.format for better error messages. Unfortunately, since it
is templated code, this results in Phobos template instantiations of
`format` in the user's code when `-debug` is used.

Given that default dub builds use `-debug`, this is a very common
problem.

Instead of removing the invariant checks, this changes it so they only
happen when testing Phobos itself.

std/algorithm/sorting.d

@@ -2332,7 +2332,7 @@ private template TimSortImpl(alias pred, R)
             }
 
             // Assert that the code above established the invariant correctly
-            version (assert)
+            version (StdUnittest)
             {
                 if (stackLen == 2)
                 {