Add dolfin.cpp.common.has_debug()

[Tormod Landet]

Some tests only work properly in debug mode where C++ dolfin_assert becomes a Python RuntimeError.

The had_debug() function should be moved to C++ from site-packages/dolfin_utils/test/skips.py

See also this related commit and issue #739

[Jan Blechta]

b577b34153a31467e19a92f0af202924866e13f3