tests: dma: chan_blen_transfer: test breakage on Nucleo-H743ZI

[mathieuchopstm]

Describe the bug

(follow-up issue to #75125)

After merge of #75245, the chan_blen_transfer test now succeeds on STM32WB0 MCUs but fails on Nucleo-H743ZI.

Relevant test log extract:

===================================================================                                                                       
START - test_tst_dma1_m2m_chan0_burst16                                                                                                   
Preparing DMA Controller: Name=dmamux@58025800, Chan_ID=4, BURST_LEN=2                                                                    
Starting the transfer                                                                                                                     
E: invalid source address                                                                                                                 
E: cannot configure the dmamux. 

The invalid source address message is emitted here: https://github.com/zephyrproject-rtos/zephyr/blob/662b9803fa9985f499d3627f96589a1ed8d2c8dc/drivers/dma/dma_stm32_bdma.c#L566-L573 where bdma_stm32_is_valid_memory_address is: https://github.com/zephyrproject-rtos/zephyr/blob/662b9803fa9985f499d3627f96589a1ed8d2c8dc/drivers/dma/dma_stm32_bdma.c#L462-L478

TL;DR: the DMA driver refuses the transfer because the source is not in SRAM4.

The regression comes from the fact that, beforehand, the .rodata and .bss segments of the test code could be relocated thanks to: https://github.com/zephyrproject-rtos/zephyr/blob/662b9803fa9985f499d3627f96589a1ed8d2c8dc/tests/drivers/dma/chan_blen_transfer/CMakeLists.txt#L10

And indeed, the Kconfig for Nucleo-H743ZI makes use of this feature: https://github.com/zephyrproject-rtos/zephyr/blob/662b9803fa9985f499d3627f96589a1ed8d2c8dc/tests/drivers/dma/chan_blen_transfer/boards/nucleo_h743zi.conf#L5-L9

Before #75245, the test looked like this: https://github.com/zephyrproject-rtos/zephyr/blob/4180d70439f599a3597b2e31753773cfa222ace2/tests/drivers/dma/chan_blen_transfer/src/test_dma.c#L25-L26

• tx_buff is static const and initialized, so it goes in .rodata and gets relocated
• rx_buff is static, not const, but uninitialized, so it goes in .bss and gets relocated After the patch, it now looks like this: https://github.com/zephyrproject-rtos/zephyr/blob/662b9803fa9985f499d3627f96589a1ed8d2c8dc/tests/drivers/dma/chan_blen_transfer/src/test_dma.c#L25-L26
• tx_buff is static, not const, and initialized, so it goes in .data and is not relocated
• rx_buff is static, not const, but uninitialized, so it goes in .bss and gets relocated

Expected behavior Test should not fail.

Impact Test breakage on Nucleo-H743ZI and potentially all boards with a Kconfig overlay setting CONFIG_DMA_LOOP_TRANSFER_RELOCATE_SECTION (i.e., mimxrt boards).

[mathieuchopstm]

Potential fix proposition: move tx_data and rx_data to a separate file, and relocate the whole file.

Appears to work on STM32WB0 and STM32H7, PR incoming.

[mathieuchopstm]

Fix proposed in #75895.