Test build.py

[jayvdb]

In order to find the problems in the Darwin build, I found it useful to start building a test suite for it.

jayvdb@10d200a

To make it efficient, we would need something like miurahr/aqtinstall#224 which has been half implemented in miurahr/aqtinstall#225. Depending on how we use aptinstall, what was added may be sufficient for efficient tests.