cm5200: Make function test command names more unique

Add "_test" to cm5200's function test command names to prevent
overlap with common, global function names.  Originally, the
"do_i2c" function test command interfered with
common/cmd_i2c.c's "do_i2c" when CONFIG_I2C_CMD_TREE was defined.

The functions were also made static as they are not globally accessed.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>