Tests: Make CMake.FileDownload invalid host name case more robust

Use a well-defined `.invalid` top-level domain instead of assuming there
is no `.png` top-level domain.  Extend the timeout of this one case to
30 seconds to give DNS lookups a chance to fail instead of timing out.

Tests/CMakeTests/FileDownloadTest.cmake.in

@@ -138,9 +138,9 @@ message(STATUS "${status}")
 
 message(STATUS "FileDownload:11")
 file(DOWNLOAD
-  badhostname.png
+  badhostname.invalid
   ${dir}/file11.png
-  TIMEOUT ${timeout}
+  TIMEOUT 30
   STATUS status
   )
 message(STATUS "${status}")