build: Adopt vtest for varnishtest

[dridi]

Here is the check list of steps to get there:

• Make the check list
• Re-start the vtest repo as vtest2, preserving the git history from varnish-cache
• Add vtest2 as a submodule of varnish-cache
• Register custom commands in vtest
  This would give us the ability to retain vtc_varnish, vtc_logexp etc here, remove them in vtest.
• Split vtest into a library and a program
  Some command line options like -i only make sense here, remove them in vtest.
• Add tooling to bundle vtest as a git submodule
  • Add automation to vendor vtest when we branch out
    In trunk we can track vtest master as a submodule, we replace it with a static copy when we create x.y branches.
  • Add automation to apply vtest patch to x.y branches
    The goal is facilitate cherry-picks from the vtest tree to the varnish-cache sub-tree
• Make varnishtest use libvtest.a
  And keep varnishtest with varnish-specific command line options.

Then we can support TLS in vtest.

[nigoroll]

Prior work: #2838

[nigoroll]

Updated plan from VDD:

• Re-start the vtest repo as vtest2, preserving the git history from varnish-cache
• Change order of TODOs, start with submodule

[nigoroll]

ping @deweerdt @vtoc