t/run.perl: to avoid repeated process spawning for *.t

t/run.perl: to avoid repeated process spawning for *.t

Spawning a new Perl interpreter for every test case
means Perl has to reparse and recompile every single file
it needs, costing us performance and development time.

Now that we've modified our code to avoid global state,
we can preload everything we need.

The new "check-run" test target is now 20-30% faster
than the original "check" target.