tests: flash: common: randomize before each test

test_flash_erase() requires that the expected[] array contains
pseudo-random data. However, the expected[] array would only
be initialized once before all tests are run using the setup
callback.

Instead, use the before() callback to randomize data before
each test in the suite, since there is otherwise no guarantee
that test_flash_erase() will be run directly after the
expected[] array has been randomized.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>