gcoap_forward_proxy: No test provided

[maribu]

Description

The module gcoap_forward_proxy has no test coverage, not even compile testing.

Steps to reproduce the issue

Run USEMODULE=gcoap_forward_proxy BOARD=atmega256rfr2-xpro make -C examples/gcoap. It will fail to compile (at least on Ubuntu/Debian systems with the prehistoric AVR toolchain) with:

**sys/net/application_layer/gcoap/forward_proxy.c:356:23: error:comparison between signed and unsigned integer expressions [-Werror=sign-compare]
     for (int i = 0; i < client_pkt->options_len; i++) {
                       ^**

Expected results

The module would compile on all platforms, at least with the toolchain used in riot/riotbuild, as it is compile tested by the CI.

Actual results

It fails to compile on some platforms.

Versions

All up to master