Added edge case test to mem.count
Added edge case test to mem.count

Some implementations break on this edge case. Thought relevant to add it.
raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig