For local running one can still tests AllTests, but for C.I. one can
test the 4 of them in parallel (and, importantly, deactivate the remote
interop tests until we fix them on Travis; while keeping the rest).

I had to make the remote tests a subclass of an abstract InteropTests
(like InteropTestsLocalSSL and InteropTestsLocalCleartext), and then
deactivate the abstract InteropTests on the 4 schemes that use it.

Eventually all samples will be tested.

Warnings will be emitted when trying to import through alpha packages.

Still needs implementing format_as_block, and switching input and output
to use sys.stdin.readline() and sys.stdout.write(line).

Also uses the .proto files for the service/stub definitions.