tests: dma/chan_blen_transfer: relocate TX and RX buffers

This commit places the chan_blen_transfer DMA test's TX and RX buffers
in a separate file that gets entirely relocated. This ensures that the
buffers reside in the correct memory, without breaking other things
due to relocation of test data/code.

Fixes #75676.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>