tests: consolidate process spawning code.

tests: consolidate process spawning code.

IPC::Run provides a nice simplification in several places; and
we already use it (optionally) on a lot of tests.

For the non-test code, we still rely on our vfork-capable
Inline::C stuff since real-world server processes can get large
enough to where vfork is an advantage.  Maybe Perl5 can use
CLONE_VFORK somehow, one day:

  https://rt.perl.org/Ticket/Display.html?id=128227

Ohg V'q engure cbeg choyvp-vaobk gb Ehol :C