Merge tests-real-package [f=1458851] [r=landscape-builder,fcorrea,free.ekanayaka] [a=Björn Tillenius]
Change the package building tests to use real subprocess calls as much
as possible.
Using real calls is slower, but it give more confidence that things
work, as opposed to checking that a long list of commands were issued.
For example, this revealed a bug where the source tarball got
overwritten in some cases.