tests: DMA loop transfer: SW handshake for mem-to-mem transfers

[lidt-coder]

Memory-to-memory transfers in the DMA loop_transfer tests did not
explicitly select a software handshake (set source/dest_handshake to 1).
On controllers that expect a hardware peripheral request for transfers,
leaving the handshake at the default HW(source/dest_handshake to 0)
value causes the DMA to stall because no physical trigger exists for M2M transfers.

Am I right? the docs specifically says that:

/**
	 * Source handshake, HW specific
	 *
	 * - 0b0 HW
	 * - 0b1 SW
	 */
	uint32_t  source_handshake :     1;
	/**
	 * Destination handshake, HW specific
	 *
	 * - 0b0 HW
	 * - 0b1 SW
	 */
	uint32_t  dest_handshake :       1;
*/