tests: py: dfu: Add variables to store dfu alt numbers for test and dummy files

This patch replaces hardcoded (i.e. 0 and 1) values passed to dfu_{read|write}
with variables.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
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Changes for v3:
- Replace per module global variables with ones defined inside a function
Changes for v2:
- None