t/common: introduce run_script wrapper for t/cgi.t
This will give us a consistent interface for running
test scripts in more performant ways while still giving
us a consistent interface to recreate real-world behavior
via spawn() (fork + execve), if needed.
The default run_mode (1) is faster and can run within the test
process with some minor adjustments to our code to avoid global
state.
This avoids the significante overhead of Perl code loading,
parsing and compilation phases.