[libc++] Fix several double-moves in the code base (#104616)

This patch hardens the "test iterators" we use to test algorithms by
ensuring that they don't get double-moved. As a result of this
har

[libc++] Fix several double-moves in the code base (#104616)

This patch hardens the "test iterators" we use to test algorithms by
ensuring that they don't get double-moved. As a result of this
hardening, the tests started reporting multiple failures where we would
double-move iterators, which are being fixed in this patch.

In particular:
- Fixed a double-move in pstl.partition
- Add coverage for begin()/end() in subrange tests
- Fix tests for ranges::ends_with and ranges::contains, which were
incorrectly calling begin() twice on the same subrange containing
non-copyable input iterators.

Fixes #100709

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