Implement 'out' direction on allocator tests

This patch adds the paths for searching for instance allocators and
makes the LUTestAllocator code run the allocator and return the results
if the direction specified is 'out'. 'out' means that the opcode will
return the result of the allocator run, instead of the allocator input
file ('in').

The patch unifies all names to refer to 'iallocator' instead of plain
allocator.

The patch also adds an example allocator that can be used for testing
this new functionality.

Reviewed-by: ultrotter