I'll expand on my statement with consideration to the first post. On a squib test you are passing a low test voltage through the squib. this is a continuity check that won't blow the bottle.
The firing circuit is 28v that is enough to fire the squib.
If during your fire test the light does not illuminate an Engineer must confirm it is the test circuit. We basically take the plug off and simulate the bottle being fired under 28v conditions and check the squib with a low voltage tester. If that is the case it can be dispatched under the test being faulty and not the squib.