tests: logalloc_test: relax test_large_allocation
tests: logalloc_test: relax test_large_allocation

test_large_allocation attempts to allocate almost half of memory.
With a buddy allocator, even if more than half of memory is free,
and even if it is contiguous, it is unlikely to be available as a
single allocation because the allocator inserts boundaries at powers-
of-two addresses.

Relax the test by allocating smaller chunks (but still the same amount,
and still with challenging sizes); allocating half of memory contiguously
is not a goal.

Also use a vector instead of a deque, and reserve it, so we don't get
intervening non-lsa allocations. I'm not sure there's a problem there
but let's not depend on the allocation patterns.
Message-Id: <d6f105c560aeb5e975fa938109a0ddb7ac7b46eb@scylladb.com>

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