refactor-for-testability (#3)

Co-authored-by: jdl <jdl@desktop>
Co-authored-by: jdl <jdl@crumpington.com>
Reviewed-on: #3

peer/header_test.go

@@ -0,0 +1,21 @@
+package peer
+
+import "testing"
+
+func TestHeaderMarshalParse(t *testing.T) {
+	nIn := header{
+		StreamID: 23,
+		Counter:  3212,
+		SourceIP: 34,
+		DestIP:   200,
+	}
+
+	buf := make([]byte, headerSize)
+	nIn.Marshal(buf)
+
+	nOut := header{}
+	nOut.Parse(buf)
+	if nIn != nOut {
+		t.Fatal(nIn, nOut)
+	}
+}