Use single test runner executable for unit tests.

This is not only much cleaner than the old hack that depended
on the tested modules not being linked in a second time from
the druntime/Phobos static libraries, but the hack obviously
also doesn't work for shared libraries.

Also unify the naming scheme and adapt the Travis config to
respect it.
Set rpath for shared library test config.

This way, the user doesn't have to set LD_LIBRARY_PATH to be
able to run the test suite. We don't want to do this in the
install config, though, as rpath has security implications.
CMake: Link shared druntime/Phobos libraries to m/pthread/dl/rt.
Build druntime/test/shared tests when BUILD_SHARED_LIBS=ON.
Travis CI: Build unittest libraries in verbose mode to avoid silence timeout.
Fold in shared library runtime updates.

This brings the testsuite for druntime/Phobos as shared libraries
to the same level as when built against static libraries.
Travis CI: Run tests with BUILD_SHARED_LIBS=ON.
CMake: Build shared library Phobos test runner with --no-as-needed.

--as-needed seems to be on by default on Ubuntu 12.04 LTS
(GNU ld (GNU Binutils for Ubuntu) 2.22). This leads to
libphobos2-ldc-unittest.so not being linked it all, and thus
the modules to test not being found.
Travis CI: Make use of YAML multi-line strings for readability.
Travis CI: Again split up release mode multilib tests.

We still hit the 50 minute timeout otherwise. The
implementation, ..., well, I'd rather not talk about this.