tests: flash: stm32: add block registers tests

Add a test of FLASH_STM32_EX_OP_BLOCK_OPTION_REG and
FLASH_STM32_EX_OP_BLOCK_CONTROL_REG extended operations for stm32f4.

It verifies that the Option Byte and Control registers are blocked
correctly. The registers can be unlock after reboot, so it is needed
to separate this test from other tests.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>