`<memory_resource>`: Conformance issues on `monotonic_buffer_resource::do_allocate`

I find 2 conformance issues in the current implementation of monotonic_buffer_resource::do_allocate.

First, it does not meet to the requirement of [mem.res.monotonic.buffer.mem]/6.

This can have observable behavior differences if the upstream resource throws type of some derived bad_alloc on failure, which is not the exception throw by the call to _Xbad_alloc in monotonic_buffer_resource::_Increase_capacity.

This can be fixed by calling to the allocation function of the upstream resource with sufficient large size and alignment (so it is expected to fail).

Second, it fails to meet the allocation function requirement (implied by [mem.res.private]/2) for zero sizes when called more than once.

This can be fixed by changing the size to 1 before the call to std::align.

Start by reading monotonic_buffer_resource::do_allocate and its _Increase_capacity path, then compare both allocation cases with the linked standard requirements. Done means the upstream failure preserves the required exception behavior and repeated zero-size allocations meet the allocation-function requirement; validate these cases with the relevant existing tests.