Update dub.sdl - mark targetType as an executable
Update dub.sdl - mark targetType as an executable

Before, dub would build the unit-threaded test runner as a library, therefore not actually executing any tests.

This modifies dub.sdl so it builds an executable.
shishkander/unit-threaded,atilaneves/unit-threaded,shishkander/unit-threaded,atilaneves/unit-threaded