memtx: use MemtxAllocator::collect_garbage in unit test

Once we start reusing read views, we won't be able to allocate and free
a tuple to trigger garbage collection in tests, because the tuple may
get attached to the last read view. Let's make the collect_garbage()
method public and use it in tests.

While we are at it, let's also rewrite the destroy() method using
collect_garbage().

Needed for #7189

NO_DOC=refactoring
NO_CHANGELOG=refactoring