We pull in vtest2 as a submodule and build varnishtest from those sources. As of
today, the sources are identical and development will continue in vtest2.

To build from git, either use "git clone -r" for the initial clone, or use "git
submodule update --init" on an existing repository. This is also explained in
changes.rst and recommended by configure if it does not find the vtest2 sources.

The vtest2 reference commit can be updated by calling "make update".

Other changes concerning varnishtest/Makefile.am are partly due to the existing
code partly not being entirely correct. For example, explicitly using ">
$(builddir)/teken_state.h" should not be necessary and built sources should be
added to BUILT_SOURCES.

We remove local copies of the varnishtest code and all a*.vtc test cases, which
now live in vtest2.

Next step in implementing #3983