Registry tests (bis)

I'm putting up the pull request already, although there's still a problem with `TestPull` and `TestPush` when trying to start containers, looks like the `/sbin/init` binary is not the right one.

stderr logs for the container look like this:

    flag provided but not defined: -g
    Usage of /sbin/init:
      -test.bench="": regular expression to select benchmarks to run
      -test.benchtime=1: approximate run time for each benchmark, in seconds
      -test.cpu="": comma-separated list of number of CPUs to use for each test
      -test.cpuprofile="": write a cpu profile to the named file during execution
      -test.memprofile="": write a memory profile to the named file after execution
      -test.memprofilerate=0: if >=0, sets runtime.MemProfileRate
      -test.parallel=1: maximum test parallelism
      -test.run="": regular expression to select tests and examples to run
      -test.short=false: run smaller test suite to save time
      -test.timeout=0: if positive, sets an aggregate time limit for all tests
      -test.v=false: verbose: print additional output

FWIW the same logic was working in the first version (#578)