Unit tests for PaddedConnectionWriter. Also broke some shared test

code out into separate classes.

test/net/sf/briar/transport/NullConnectionEncrypter.java

@@ -0,0 +1,22 @@
+package net.sf.briar.transport;
+
+import java.io.IOException;
+import java.io.OutputStream;
+
+/** A ConnectionEncrypter that performs no encryption. */
+class NullConnectionEncrypter implements ConnectionEncrypter {
+
+	private final OutputStream out;
+
+	NullConnectionEncrypter(OutputStream out) {
+		this.out = out;
+	}
+
+	public OutputStream getOutputStream() {
+		return out;
+	}
+
+	public void writeMac(byte[] mac) throws IOException {
+		out.write(mac);
+	}
+}