Splitting unit-test on per-file executable

Works better with CTest.
Each file named test_*.cpp becomes a test.
Some test files are excluded if AHMED is not present.
Some tests require additional file objects. These files are included as sources
for now, but should made into a static library if possible. Object libraries
are likely still too recent a cmake feature to use yet.

Implements #110
Meshes are copied using cookoff functionality
Test fixtures are now static libraries

The fixtures are only compiled once.
Closes #110
Merge pull request #117 from bempp/rsdt/issue/110

Rsdt/issue/110