Perhaps you could use the I2C general call (address 0x00) sending two bytes (e.g.) -- the first byte is a "safing" command byte (e.g. 0xA5 indicates a command that change the TWAR) and a second which is that address.
Once the address is changed and any re-initializations required are complete, you can do your address-specific testing.
When you're ready to switch addresses your master issues another general call with a new address etc.