Automated Test Suites for Controllers - By Forcing I/O

I am looking to set up a testing system to automatically test a PLC. I don't want to use PLC simulation, I want to have the PLC operating in it's real-world environment, i.e. the test system is forcing IO as if the real world environment was changing. After I make significant changes to the PLC program, I want to set the tester to run overnight and run every test again to prove to the customer that nothing has inadvertently been broken. In the morning I expect to see a whole lot of green lights down the screen.

I'm basically thinking of an I/O rack that is controlled by PC, each output on the tester is connected to an input on the EUT (Equipment Under Test), each input on the tester connected to an output on the EUT. I only need clean-contact relays (drive and sense) and 4-20mA loops (drive and sense). 3 Phase 5A motor current simulators would be handy but I think that will make it significantly harder to achieve.

A Modbus Master incorporated would be great, so I can read a ton of internal registers from the EUT to correctly observe it's status.

I'm thinking of a higher level language interpreter to describe the test cases, e.g.:

Definitions (constant for entire program in EUT):
Force DIN 1 = Close Dout 1
SEQUENCE_ERROR = Modbus Input 58

Test Suite 1:
Force DIN 1 TRUE
Force DIN 2 TRUE
Force DIN 4 TRUE
Expect Fault SEQUENCE ERROR.

Does anyone know of good programs or systems for achieving such a test suite? Obviously price is important, but I am interested in all options.

Thanks for your help.
Regards,
Scott Critchley