oeqa/oetest: oeSDKTest when run a command redirect env output to null
Some tests are failing because the eSDK env load script shows a help
message on the output so redirect this to /dev/null.
There was a discussion with Paul Eggleton [1], he don't want to have env
variables to change the behaviour of what the env script needs to print
so redirect the output in oeSDK.run() method. I didn't agree because
it can hides another messages like error ones and with the variable you
can control what prints or not.
