CI: vtest: Rely on VTest2 to run regression tests

CI: vtest: Rely on VTest2 to run regression tests

VTest2 (https://github.com/vtest/VTest2) was released and is a remplacement
for VTest. VTest was archived. So let's use the new version now.

If this commit is backported, the 2 following commits must also be
backported:

 * 2808e3577 ("REGTESTS: Explicitly allow failing shell commands in some scripts")
 * 82c291124 ("REGTESTS: Make the script testing conditional set-var compatible with Vtest2")

(cherry picked from commit 6a18d28ba20439a94ec38e6670e1b7b2a864bde8)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 1e863408430801b9697737b08d30dc8ae682ca3f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>