i2c: testunit: add support for block process calls

Devices offering SMBus block process calls are rare, so add it to the
testunit. This is also a good test case for testing proper
I2C_M_RECV_LEN flag handling of I2C bus masters emulating SMBus.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>