Ensure that a clean environment is used to launch Nginx in tests

This avoids potential issues when running the tests in environments which
might set variables which Nginx tries to read on startup. For example,
the $NGINX variable was being picked from the Travis-CI build matrix
configuration.

t/preamble

@@ -40,7 +40,7 @@ rm -f "${NGINX_CONF}" "${NGINX_PID}"
 mkdir -p "${PREFIX}/logs"
 
 function nginx () {
-	"${PREFIX}/sbin/nginx" "$@"
+	env - PATH="${PATH}" "${PREFIX}/sbin/nginx" "$@"
 }
 
 function nginx_conf () {