Redirect the output from script for kernel tests.

Currently, all the output from the script is going into stdout.
When a build breaks the stderr file is used for telling breakage
reason and a snippet from this file appears in emails. Right now
only the .zip downloading output goes to the stderr that unnecessarly
fills up the file. Hence, redirect all test results to stderr and
supress wget output, so that only useful information gets into email
snippets.

BUG=b:29003489
TEST=Tested locally

Change-Id: I1719ec5993c5819d509e933f543ec3c32260e92e