Script unit tests

[wneirynck]

An important part of coding is testing. Since MonkeyCI build scripts are actually small programs, we also need a way to verify their functionality. We should support script unit tests. These should be included in the build script (e.g. build_test.clj) and verify the build script integrity. The tests can be run from the cli, but would also automatically run when triggering a build.

Tests should run in a sandbox, where jobs are faked, although the user can provide custom job results. It is however not possible for us to fake everything: if the build script contains some custom piece of code that performs a side effect, it is up to the user to make sure this does not happen in tests. To this end, the build runtime will contain a flag that indicates whether the script is run in test mode or not.

Should a script test fail, the build will also fail.