REGTESTS: ssl: Move all the SSL certificates, keys, crt-lists inside "certs" directory

Move all these files and others for OCSP tests found into reg-tests/ssl
to reg-test/ssl/certs and adapt all the VTC files which use them.

This patch is needed by other tests which have to include the SSL tests.
Indeed, some VTC commands contain paths to these files which cannot
be customized with environment variables, depending on the location the VTC file
is runi from, because VTC does not resolve the environment variables. Only macros
as ${testdir} can be resolved.

For instance this command run from a VTC file from reg-tests/ssl directory cannot
be reused from another directory, except if we add a symbolic link for each certs,
key etc.

 haproxy h1 -cli {
   send "del ssl crt-list ${testdir}/localhost.crt-list ${testdir}/common.pem:1"
 }

This is not what we want. We add a symbolic link to reg-test/ssl/certs to the
directory and modify the command above as follows:

 haproxy h1 -cli {
   send "del ssl crt-list ${testdir}/certs/localhost.crt-list ${testdir}/certs/common.pem:1"
 }

.github/h2spec.config

@@ -19,7 +19,7 @@ defaults
 
 frontend h2
     mode http
-    bind 127.0.0.1:8443 ssl crt reg-tests/ssl/common.pem alpn h2,http/1.1
+    bind 127.0.0.1:8443 ssl crt reg-tests/ssl/certs/common.pem alpn h2,http/1.1
     default_backend h2b
 
 backend h2b