tests: i2c_ram: Add fixture for i2c_ram testing

i2c_ram expects that a ram-like part is connected to an i2c bus and
therefore requires a hardware fixture setup much like spi_loopback.

Adds a fixture that must be designated when running twister tests on a
board with -X i2c_ram.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>