xilinx: Fix attributes_test.ys
xilinx: Fix attributes_test.ys

This test pretty much passes by accident — the `prep` command runs
memory_collect without memory_dff first, which prevents merging read
register into the memory, and thus blocks block RAM inference for a
reason completely unrelated to the attribute.

The attribute setting didn't actually work because it was set on the
containing module instead of the actual memory.

YosysHQ/yosys,SymbiFlow/yosys,SymbiFlow/yosys,antmicro/yosys,YosysHQ/yosys,cliffordwolf/yosys,SymbiFlow/yosys,YosysHQ/yosys,antmicro/yosys,antmicro/yosys,SymbiFlow/yosys,cliffordwolf/yosys,YosysHQ/yosys,YosysHQ/yosys,antmicro/yosys,cliffordwolf/yosys,cliffordwolf/yosys,antmicro/yosys,cliffordwolf/yosys,SymbiFlow/yosys,cliffordwolf/yosys,SymbiFlow/yosys,cliffordwolf/yosys,antmicro/yosys,SymbiFlow/yosys,SymbiFlow/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,antmicro/yosys,antmicro/yosys,YosysHQ/yosys,cliffordwolf/yosys