Add block-matrix code as regression test

[eschnett]

I added a new regression test based on my block-matrix code.

[hkaiser]

Eric, we would be happy to add your test if you please:

• add license and copyright to all files
• let main() return hpx::util::report_errors(); to report any detected problems back to the testing environment
• guard all gcc specifics appropriately (for instance __building_unreachable() and #include <unistd.h>), remember, HPX is meant to be built using gcc, icc, clang, and msvc.
• having HPX_HAVE_CXX11 is not the right way to protect this test from being build on non-c++11 platforms, please use more specific constants like CXX_HAVE_CXX11_RVALUE_REFERENCES, etc. it could also be that we don't even have an appropriate test available for some of the features you use (for instance std::initializer) - please see hpx/CMakeLists.txt, starting at line around 1023

[eschnett]

Will do.

By the way, the HPX_HAVE_CXX11 is from tests/regressions/actions.

[eschnett]

Done.