[gcc r17-2716] libstdc++: Robustify flat_map::insert_range move counter test

https://gcc.gnu.org/g:b71686db1fdc589da34f80460f054407c1f54990

commit r17-2716-gb71686db1fdc589da34f80460f054407c1f54990
Author: Patrick Palka <[email protected]>
Date:   Sun Jul 26 09:58:58 2026 -0400

    libstdc++: Robustify flat_map::insert_range move counter test
    
    Testing r17-2704-gf678da4a9930c8 on the 16 branch, which doesn't
    implement P0401 std::allocate_at_least, revealed that the added test
    isn't robust to std::vector initial capacity.  The test needs to reserve
    sufficient capacity upfront so that reallocation doesn't happen during
    insert_range which would skew the move counter.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/23_containers/flat_map/1.cc (test14): Reserve
            sufficient capacity for the underlying containers before
            calling insert_range.

Diff:
---
 libstdc++-v3/testsuite/23_containers/flat_map/1.cc | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
index d53e56a1b717..c3ad2883e65f 100644
--- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
@@ -433,6 +433,12 @@ test14()
     {counter(2), counter(20)},
     {counter(3), counter(30)},
   };
+
+  auto [keys,values] = std::move(m).extract();
+  keys.reserve(std::size(r));
+  values.reserve(std::size(r));
+  m.replace(std::move(keys), std::move(values));
+
   moves = 0;
   m.insert_range(std::sorted_unique, std::views::as_rvalue(r));
   VERIFY( moves == 6 );