tests/fetch: Allow to work on python 2.6

Currently the fetch tests fail on python 2.6 since check_output
doesn't exist on this version of python. Use bb.process instead
to avoid this problem.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
fetch2/utils: Clean up imports

Move various random imports to the start of the modules as cleanup
and avoid an import issue with bb.process on python 2.6.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
test/fetch: Split the network tests from the rest of the fetcher tests

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
test/fetch: Allow the conditional network tests to work under python 2.6

The skipIf decorator is unavailable under python 2.6 and finding a syntax
works there is hard. This patch does allow network tests under 2.6.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>