Added a test

It's still pretty bad. I'll add more, I promise

Author: joliv

spark_test.go

@@ -0,0 +1,16 @@
+package spark
+
+import (
+	"testing"
+)
+
+func TestSpark(t *testing.T) {
+	var (
+		// Wish I could use constant slices...
+		in  = []float64{1, 2, 3, 4, 5, 6, 7, 8}
+		out = "▁▂▃▄▅▆▇█"
+	)
+	if x := Spark(in); x != out {
+		t.Errorf("Spark(%v) = %v, want %v", in, x, out)
+	}
+}