Print the result of command when test is failed

When ConnectivityManagerTest#testGetMultipathPreference() is
failed, it only prints "Unexpected format from cmd netpolicy" and
it's not helpful.
Print the result of command can help people to know what's the
difference between expected result and actual result.

Bug: 187921745
Test: atest CtsNetTestCases:ConnectivityManagerTest#testGetMultipathPreference
Change-Id: I63cd4b043e7c70775156aa5244d25ee2268f9f32
1 file changed