Unit tests for PaddedConnectionWriter. Also broke some shared test

code out into separate classes.

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

@@ -0,0 +1,29 @@
+package net.sf.briar.transport;
+
+import java.io.EOFException;
+import java.io.IOException;
+import java.io.InputStream;
+
+/** A ConnectionDecrypter that performs no decryption. */
+class NullConnectionDecrypter implements ConnectionDecrypter {
+
+	private final InputStream in;
+
+	NullConnectionDecrypter(InputStream in) {
+		this.in = in;
+	}
+
+	public InputStream getInputStream() {
+		return in;
+	}
+
+	public void readMac(byte[] mac) throws IOException {
+		int offset = 0;
+		while(offset < mac.length) {
+			int read = in.read(mac, offset, mac.length - offset);
+			if(read == -1) break;
+			offset += read;
+		}
+		if(offset < mac.length) throw new EOFException();
+	}
+}