WIP: Out of tree system tests

This MR replaces !68 (closed), submitted by @pemensik. Quoth he:

I started playing with out-of-tree builds recently. But I could not find an easy way to start tests in bin/tests/system with build made in external directory. This change will make much more simple to run tests against extra build. After external build is complete, just make symlink into source bin/tests/system conf.sh from the build directory. Then tests can be started from source directory without additional changes.

This is especially useful when testing different build configuration from the same sources.

With these changes, plus a few more I put in, you can copy conf.sh from the build tree to the source tree, then run "make -j8 test" in the source tree, and the tests pass.

Closes #114 (closed)