libstdc++: Fix tests for COW std::string [PR 96088]

The expected number of allocations is different when copying COW
strings.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/96088
	* testsuite/23_containers/unordered_map/96088.cc: Adjust
	expected number of allocations.
	* testsuite/23_containers/unordered_set/96088.cc: Likewise.