Depending on the host machine, a default `par` value can spawn up to 15 script verification threads for each node.
Running the functional test suite with default `par` can exhaust file descriptors or hit other resource limits when many threads are spawned.
These threads are mostly idle and the same code paths are executed with a value of `par=2`.
Limit this to 2 for functional tests that do not override the default option.
Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>
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