integration: buildfile: updated to new NewBuildFile

This patch fixes the integration tests for buildfiles to
use the new function signature for NewBuildFile (which requires
the macros parameter). The macros parameter used is nil, to
preserve compatibility.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
server: buildfile: added unit tests

This patch adds unit tests to buildfile, specifically it adds unit
tests for `macroSubstitution`, which was added in this patchset.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)