test: efi_capsule: refactor efi_capsule test

Current efi capsule python tests have much code duplication.
This commit creates the common function
in test/py/tests/test_efi_capsule/capsule_common.py,
aim to reduce the code size and improve maintainability.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>