Generally , no, you determine wall thickness as per the design requirements. Most designs allow for the extra stress in the hydrotest case. E.g if you need say 10 bar, then but pipe rated for 10 bar. The fact you test it at 12.5 or 15 is included in the design, i.e. you don't buy 15 bar pipe if 15 is the hydrotest pressure.
UPVC might be only 1.25 times rated working pressure / design pressure. You would need to check manufacturers specifications / data sheets.
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