tests: move common stuff into a common mod

tests/common.rs

@@ -0,0 +1,12 @@
+#[allow(unused)]
+pub fn hash<const N: usize>(px: [u8; N]) -> u8 {
+    let r = px[0];
+    let g = px[1];
+    let b = px[2];
+    let a = if N >= 4 { px[3] } else { 0xff };
+    let rm = r.wrapping_mul(3);
+    let gm = g.wrapping_mul(5);
+    let bm = b.wrapping_mul(7);
+    let am = a.wrapping_mul(11);
+    rm.wrapping_add(gm).wrapping_add(bm).wrapping_add(am) % 64
+}