Make sure VTCs run against all binaries

Lexicographic order is fine, but can lead to interesting things like:

    ================================================================
    Testsuite summary for Varnish trunk
    ================================================================
    # TOTAL: 571
    # PASS:  565
    # SKIP:  6
    # XFAIL: 0
    # FAIL:  0
    # XPASS: 0
    # ERROR: 0
    ================================================================
    Making check in varnishtop
      CC       varnishtop-varnishtop.o
      CC       varnishtop-varnishtop_options.o
      CCLD     varnishtop
    Making check in etc
    Making check in doc
    Making check in graphviz
    Making check in sphinx
    Making check in man

In other words, picking `varnishtop` from somewhere else in the PATH.
Since SUBDIRS are processed sequentially, this is change is enough to
cover parallel builds too.

While at it, polish u4.vtc a bit: get rid of the shell process.

bin/Makefile.am

 #
 
+# XXX: varnishtest MUST always be built last
+
 SUBDIRS = \
 	varnishadm \
 	varnishd \
@@ -7,5 +9,5 @@ SUBDIRS = \
 	varnishlog \
 	varnishncsa \
 	varnishstat \
-	varnishtest \
-	varnishtop
+	varnishtop \
+	varnishtest

bin/varnishtest/tests/u00004.vtc

@@ -7,7 +7,7 @@ server s1 {
 
 varnish v1 -vcl+backend "" -start
 
-process p1 {varnishtop -1 -n ${v1_name} > /dev/null} -start
+process p1 {exec varnishtop -1 -n ${v1_name} >/dev/null} -start
 
 client c1 {
 	txreq
@@ -16,4 +16,4 @@ client c1 {
 
 process p1 -wait
 
-process p2 {varnishtop -1 -n ${v1_name} > /dev/null} -start -wait
+process p2 {exec varnishtop -1 -n ${v1_name} >/dev/null} -start -wait