Fix incorrect test root in build.zig

src/main.zig

@@ -15,7 +15,7 @@ pub fn main() anyerror!void {
 }
 
 test {
-    // Empty test is here to run all others (for now).
+    _ = @import("./mem.zig");
 }
 
 fn run(event_loop: *sys.EventLoop, graphics: *sys.GraphicsContext) anyerror!void {