test/cmd/wget.c: Fix loadaddr rewrite

test/cmd/wget.c: Fix loadaddr rewrite

The $loadaddr variable is a hexadecimal value, not a string, it must be
assigned using env_set_hex(). This may break follow up tests, like the
dm_test_cmd_hash_md5 in CI. To avoid any interference with other tests,
set $wgetaddr variable which is specific to this test and use it in the
test.

Fixes: 20f641987f83 ("test/cmd/wget.c: move net_test_wget() to the cmd test suite")
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>