I do not understand.
Is this true?
You want to test LM7805 and LM7905 from a bench power supply that has a output of 0V and +27V but no -27V.
Testing LM78XX is easy you know how using +27V. I would use a socked to plug the LM78xx into. Put a capacitor from input to ground and a capacitor from output to ground. Use a meter to read the output voltage. Maybe you want a small load.
To test LM79XX do the same as above. The pin out is different. Connect the ground of the bench power supply to "input voltage" on LM79xx. Connect +27V from the bench supply to "ground" of the LM79xx.