drivers: flash: flash_simulator: Erase flash first

With native_posix, a flash.bin file is created when running the tests.
If the tests are re-run, they fail since the flash is no-longer erased.

Before running the tests, erase the flash so that the 'read' test can
work regardless of the previous state.

Fixes: 25171
Signed-off-by: Simon Glass <sjg@chromium.org>