Renovate varnishtest a bit.

Define a macro for all programs, so they can be overwritten from
the command line.

Change '-i' to mean "You're in a source tree":

	Find top of build-tree by reading Makefile

	Set $PATH which can find all programs
Define a "vmod_xxx" macro for each vmod and redefine it suitably
when -i is given.

This allows the testcases to be written:
	import ${vmod_directors};

Add a feature for ${topbuild} and test it in the remaining to tests
which use ${topbuild}.  I don't see any obvious way to write those
two cases without needing ${topbuild}.

I belive we are now very close to being able to run our testcases
out of tree.