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.
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.
Unify test naming scheme; update Travis config to match new test runner.
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.
Travis CI: Again split up release mode multilib tests.

We still hit the 50 minute timeout otherwise.
Fold in 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.