Merge branch 'feature/FS1726-complex-for-fp-only' into 'master'

Test complex<T> for floating point T only

The standard leaves (the effect of instantiating) complex<T> for more
general T unspecified.

While we're at it, kill duplicate tests and add long double to the list.

This fixes bug flyspray/FS#1726: Now also with clang 3.7 and libstdc++ 4.9.2 on Debian 8.2, the test fvectortest can be compiled again.

See merge request !11
(cherry picked from commit 627fc0b0)

Signed-off-by: Steffen Müthing <muething@dune-project.org>