Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
Problem: Problem restoring 'packpath' in session.
Solution: Let "skiprtp" also apply to 'packpath'.
d23b714d8b
Port Test_mksession_skiprtp() to lua functional test.