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.