Stainless steel, hydrostatic test.

If tap water fails to do your hydrostatic pressure test, I have serious doubts about the Stainless Steel you are using and/or your tab. You can bring your chlorine level down by letting it sit exposed to air and sunlight, just give the chlorine time and space to evaporate. Doing your test with a different source, can also be a way out: rainwater, oil, Mind formation of algae.

You are right. Generally 40 ppm of chloride is considered safe enough. Best is to use de-mineralised water, which is easily available. Else drain the vessel at the earliest and rinse it thoroughly with (low chloride ppm) water.

Discuss the matter with the Engineer/Surveyor from the company that provides burst/collapse insurance cover on the vessels.