Previsouly whenever the target list changed, _ALL_ tests were
recompiled/rerun. Now only actually new targets will be run/compiled
while removed ones will simply be discarded.

This partially reverts commit 6356a2c1.
The script is still added, but the install requires for numa is removed
and the import delayed.

This cloes #29

using > 1 process results in hard to debug issues

This reverts commit ff1b19df.

refactor la backends discovery/usage

* common.so -> _common.so, to allow for magic in __init__.py
* create module infrastructure in build tree by symlink during cmake

reduces spurious re-runs by only updatin gthe builder defition
when it actually changed

- now shows which new/revomed tests trigger rebuilding
- no longer limited to building with ninja

This reverts commit b50a7d9b.
It was actually missing in dune-gdt, plus typo ridden too.