Use the recently added assert_subprocess_success [1] helper function
more widely.
Add run_custom_sharder helper.
Add container-sharder key to the ProbeTest.configs dict.
[1] Related-Change: I9ec411462e4aaf9f21aba6c5fd7698ff75a07de3
Change-Id: Ic2bc4efeba5ae5bc8881f0deaf4fd9e10213d3b7