Bug 18731 – Link only with druntime for the runnable tests if Phobos isn't needed

All 1283 fail_compilation tests compile in ~11s on my machine. The 774 compilable tests are slightly slower with ~25s ... and you don't want to know how long runnable takes :/ So the runnable tests are the real bad boys. (numbers are taken with an i7 4th Gen with full parallelism) Now a majority of the tests doesn't even need Phobos, so a simple optimization could be to only link druntime if nothing from Phobos is imported. At the moment DRuntime has a size of 9.8M whereas Phobos clocks in with 64M (including druntime). So just using DRuntime would mean a lot less work for the linker. At the moment this the linking is defined in `test/Makefile` like `DFLAGS+=-defaultlib=libphobos2.so`, now in theory it would be as easy as replacing this with `DFLAGS+=-defaultlib=libdruntime.so.a`, in practice more work is required because it needs to be done only for the tests that don't use Phobos (or all tests using Phobos need to be moved to Phobos)

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