Use a well-defined `.invalid` top-level domain.  This follows the
pattern from commit 65a3c682 (Tests: Make CMake.FileDownload invalid
host name case more robust, 2020-01-21, v3.17.0-rc1~120^2).

Add the invalid name to `ENV{no_proxy}` because DNS lookup failure is
expected.  This follows the pattern from commit c0b1f5f3 (Tests: Add
host to ENV{no_proxy} when DNS lookup failure is expected, 2021-01-29,
v3.20.0-rc1~79^2).