test: Improve layout of helper binaries

This patch moves the test helper binaries to a "one directory per
helper" layout, and also makes them to be ignored in the coverage build
instead of all builds.

With this change, "go build ./..." will build all binaries including the
test helpers, which helps make sure that module manage automation also
considers them. In particular, this makes "go mod tidy" work fine.

test/util/lib.sh

@@ -101,26 +101,29 @@ function chamuyero() {
 }
 
 function generate_cert() {
-	go run ${UTILDIR}/generate_cert.go "$@"
+	( cd ${UTILDIR}/generate_cert/; go build )
+	${UTILDIR}/generate_cert/generate_cert "$@"
 }
 
 function loadgen() {
-	go run ${UTILDIR}/loadgen.go "$@"
+	( cd ${UTILDIR}/loadgen/; go build )
+	${UTILDIR}/loadgen/loadgen "$@"
 }
 
 function conngen() {
-	go run ${UTILDIR}/conngen.go "$@"
+	( cd ${UTILDIR}/conngen/; go build )
+	${UTILDIR}/conngen/conngen "$@"
 }
 
 function minidns_bg() {
-	( cd ${UTILDIR}; go build minidns.go )
-	${UTILDIR}/minidns "$@" &
+	( cd ${UTILDIR}/minidns; go build )
+	${UTILDIR}/minidns/minidns "$@" &
 	MINIDNS=$!
 }
 
 function fexp() {
-	( cd ${UTILDIR}; go build fexp.go )
-	${UTILDIR}/fexp "$@"
+	( cd ${UTILDIR}/fexp/; go build )
+	${UTILDIR}/fexp/fexp "$@"
 }
 
 function timeout() {