Improve test support

This depends on rock-core/autobuild#15

This pull request adds final touches to testing using autoproj.

1. it adds fine-grained control over which packages should have test enabled, with
```
autoproj test --enable[=PKG,PKG,PKG]
autoproj test --disable[=PKG,PKG,PKG]
```
(both will enable or disable tests for all packages if called without arguments). In addition,
```
  autoproj test --list
```
will show which packages have tests available and which have tests enabled.

2. it adds the ability to restrict some dependencies to packages for which tests are enabled, and also gives access to whether tests are enabled in the autobuild file. The former is to avoid installing a lot of packages that most user won't ever need, and the latter is to disable compiling tests when unneeded in packages where they are expensive.