Refactor test compilation code into correct script location

[seanmakesgames]

          > I'm trying to figure out where to put the ctest build and run commands, and it looks like maybe the libexec script files, but they are kind of mixed?

Hmm, good question. I don't think this should be published in the gem, so I would refrain from having it in libexec. I think a couple of shell files inside of test would be OK for now, to be called by some Makefile targets and GH workflow steps (but not ext/libv8-node/builder.rb)

(excluding the latter because e.g I don't want people to need cmake when installing from source)

Originally posted by @lloeki in #37 (comment)