Add unit test for wxWebRequest

The environment variable WX_TEST_WEBREQUEST_URL has to
be set to https://httpbin.org or a local instance of it
to be run.

tests/test.bkl

@@ -70,6 +70,7 @@
             misc/typeinfotest.cpp
             net/ipc.cpp
             net/socket.cpp
+            net/webrequest.cpp
             regex/regextest.cpp
             regex/wxregextest.cpp
             scopeguard/scopeguardtest.cpp