Tests: Fix CMake.FileDownload test in presence of proxy

We do not actually need to contact any real http servers.
The one attempt we make is to an intentionally bad domain name.
Unset any proxy configuration that may change behavior.

Tests/CMakeTests/FileDownloadTest.cmake.in

@@ -1,3 +1,8 @@
+# We do not contact any real URLs, but do try a bogus one.
+# Remove any proxy configuration that may change behavior.
+unset(ENV{http_proxy})
+unset(ENV{https_proxy})
+
 set(timeout 4)
 
 if(NOT "@CMAKE_CURRENT_SOURCE_DIR@" MATCHES "^/")