Whilst you could use the past cont passive or the past simple passive in the example you gave, to say they are interchangeable is incorrect as they do not have an identical meaning.
The machines were being tested at 4 o'clock -
This can be two different things:
- The machine testing was in progress at 4pm (though it started before 4pm)
- The plan was to start testing the machines at 4pm but this may not have actually happened.
The machines were tested at 4pm -
This has just one meaning: they started testing the machines at 4pm and that action is now completed.
Incidentally, the difference between them is just the same as it would be in the active voice so the fact this is passive makes no difference to the use of the tenses. Important always to remember that passive in itself is NOT a tense, merely a voice, ie a different aspect or viewpoint.
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