test/py: spi: Add tests for SPI flash device

Add test cases for sf commands to verify various SPI flash operations
such as erase, write and read. It also adds qspi lock unlock cases.
This test relies on boardenv_* configurations to run it for different
SPI flash family such as single SPI, QSPI, and OSPI.

Signed-off-by: Love Kumar <love.kumar@amd.com>