test: Write the SPI image to the persistent-data directory

It is annoying to have disk images in the source directory since it
clutters up the working space.

Move spi.bin (used by the SPI tests) into the persistent-data
directory, update the driver and add a comment.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

test/dm/sf.c

@@ -27,10 +27,12 @@ static int dm_test_spi_flash(struct unit_test_state *uts)
 	uint map_size;
 	ulong map_base;
 	uint offset;
+	char pathname[256];
 	int i;
 
 	src = map_sysmem(0x20000, full_size);
-	ut_assertok(os_write_file("spi.bin", src, full_size));
+	ut_assertok(os_persistent_file(pathname, sizeof(pathname), "spi.bin"));
+	ut_assertok(os_write_file(pathname, src, full_size));
 	ut_assertok(uclass_first_device_err(UCLASS_SPI_FLASH, &dev));
 
 	dst = map_sysmem(0x20000 + full_size, full_size);

test/py/tests/test_ut.py

@@ -42,7 +42,7 @@ def test_ut_dm_init(ubman):
         with open(fn, 'wb') as fh:
             fh.write(data)
 
-    fn = ubman.config.source_dir + '/spi.bin'
+    fn = ubman.config.persistent_data_dir + '/spi.bin'
     if not os.path.exists(fn):
         data = b'\x00' * (2 * 1024 * 1024)
         with open(fn, 'wb') as fh: