tests: i2s_speed: Increase the RX buffer count

The NXP I2S driver queues 2 receive buffers to avoid receive overflows.
Allocate an extra block so we do not see test failures due to allocate
failures

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>

tests/drivers/i2s/i2s_speed/src/test_i2s_speed.c

@@ -45,7 +45,12 @@ static int16_t data_r[SAMPLE_NO] = {
 
 #define BLOCK_SIZE (2 * sizeof(data_l))
 
-K_MEM_SLAB_DEFINE(rx_0_mem_slab, BLOCK_SIZE, NUM_BLOCKS, 32);
+/*
+ * NUM_BLOCKS is the number of blocks used by the test. Some of the drivers,
+ * e.g. i2s_mcux_flexcomm, permanently keep ownership of a few RX buffers. Add a few more
+ * RX blocks to satisfy this requirement
+ */
+K_MEM_SLAB_DEFINE(rx_0_mem_slab, BLOCK_SIZE, NUM_BLOCKS + 2, 32);
 K_MEM_SLAB_DEFINE(tx_0_mem_slab, BLOCK_SIZE, NUM_BLOCKS, 32);
 
 static void fill_buf(int16_t *tx_block, int att)