Fix ranges_rotate.pass.cpp complexity checks

The complexity is "at most N swaps" _for each invocation of `rotate`_, but the tests currently assert that the total number of swaps for N calls is at most N. The standard allows that to be N squared, so the test is either requiring more than the standard (and the comment in the test) promises, or somebody just forgot to reset the counter on each iteration.

If the intent really is to verify the libc++ guarantee, not the standard one, then shouldn't `expected` be set to N/2 i.e. 5, since that's what libc++ actually does?

Either way, to be portable to other implementations, swaps should be reset on each loop. If you want to guard that with `#ifndef _LIBCPP_VERSION` it would not change the test for libc++.

Author: jwakely

libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/ranges_rotate.pass.cpp

@@ -173,6 +173,7 @@ constexpr bool test() {
       auto end   = adl::Iterator::TrackSwaps(in.data() + in.size(), swaps);
 
       for (std::size_t mid = 0; mid != input.size(); ++mid) {
+        swaps = 0;
         std::ranges::rotate(begin, begin + mid, end);
         assert(swaps <= expected);
       }
@@ -186,6 +187,7 @@ constexpr bool test() {
       auto range = std::ranges::subrange(begin, end);
 
       for (std::size_t mid = 0; mid != input.size(); ++mid) {
+        swaps = 0;
         std::ranges::rotate(range, begin + mid);
         assert(swaps <= expected);
       }