Define a CMake target for every subdirectory of test/functional that
contains functional tests, and a functionaltest_parallel target that
depends on all those targets, allowing multiple test runners to run in
parallel.
On CI, use at most 2 parallel test runners, as using more may increase
system load and make tests unstable.
Problem: :trust is executed even when inside false condition.
Solution: Make skip_cmd() return true for CMD_trust, as ex_trust() does
not handle eap->skip itself.
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.
