Rename OpTestAllocator.allocator to iallocator

This makes the OpCode more consistent with the other opcodes. The
downside is incompatibility when upgrading from 2.6, but since this is
a test opcode it shouldn't be problematic.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>