Build constraints must be added to every file. We only want to build
this package when the `integration` tag is set.
Without this, every build prints this warning:
# github.com/osbuild/osbuild-composer/cmd/osbuild-image-tests
runtime.main_main·f: function main is undeclared in the main package
We need to use different values for path constants when running the tests
on the Travis CI. This is the first step to achieve this.
Note that this commit may be reverted when Travis CI is dropped.