test/cmd/wget.c: move net_test_wget() to the cmd test suite

test/cmd/wget.c: move net_test_wget() to the cmd test suite

Since net_test_wget() is testing a command and is in test/cmd it should
be in the 'cmd' test suite, not 'lib'.

Saving and restoring the values of the environment variables that the
test manipulates is necessary to avoid a regression when running the
whole ut test suite. A minimal reproducer is:

 $ ./u-boot -T -c "ut cmd net_test_wget; ut dm dm_test_eth_act" | \
     grep -E "(Test:|Failures:)"

Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>