app_update: fix test failure for get_app_elf_sha256 test
app_update: fix test failure for get_app_elf_sha256 test

Commit fc03161f updated esp_ota_get_app_elf_sha256 to store and return
X number of bytes of the hash, but the test case still expected 64 bytes.

Updated test case to use CONFIG value for expected length.

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf