With the draft to support both gob and protobuf as serialisation formats
for Packer, along with the SDK changes that propel them, we add a series
of tests that make sure the logic that picks which protocol is solid and
functional.
These tests rely on building several versions of the tester plugin, with
and without protobuf support, to then install them in the tests as
needed to test the logic of Packer using packer build with them, and
templates that require multiple plugins.
The lib name for the common components for writing packer_test suites
was not clear, and did not follow the convention established in Packer
core and plugins.
Therefore this commit does two things: first the lib is renamed into
common as to follow this convention, and clearly document which
components are common to all tests.
Also checkers are placed in a subpackage of common, common/check, so
that it is clearer what is meant to be used as checks for a command's
execution status after it's been run, as part of Assert.
