Because Pakistan's missile program has reached the heights of maturity and sophistication.
"Testing" means "validation of theoretical measures" ....
Nations who test their missiles a lot usually have shitty missile programs (like india) and have a long way to go in achieving total confidence in deploying their missile program.
As your missile program matures, you tend to test less.
When was the last time U.S tested any ballistic missile? Exactly.
Current inventory of Pakistan missile forces is up to the mark, ready to be fired, well-tested, matured, and deployed with confidence.
Unless Pakistan comes up with a new project...such such a longer range weapon---there is little need to test anything.
But I don't believe Pakistan will develop a longer range missile. They don't need to. Pakistani missiles already cover entire Indian Subcontinent and even beyond.
Why would Pakistan will be bothered by an indian nirbhay?
india is working on stealthy, terrain hugging nuclear cruise missiles? Oh wow---Pakistan achieved that capability a
DECADE ago.