Rename the generated root module for "dub test" to "dub_test_root".
Rename the generated root module for "dub test" to "dub_test_root".

This is clearer than test_main (after all, it doesn't necessarily contain 'main()') and also reduces the risk of name clashes.

y12uc231/dub,D-Programming-Language/dub,Geod24/dub,dreamsxin/dub,jelmansouri/dub,Abscissa/dub,xentec/dub,Flamaros/dub,grogancolin/dub,gedaiu/dub,jeanbaptistelab/dub,p0nce/dub,alexeibs/dub,Abscissa/dub-data-mod,schuetzm/dub,nazriel/dub,rjframe/dub