Fixed linker error when building unit tests for projects that depend on dxml.
Projects that depended on dxml and built their own unit tests were failing to build due to linker errors. For whatever reason, some symbols from dxml's unit tests weren't linking in properly, but really they shouldn't have been being built in the first place, since they're dxml's unit tests, not the unit tests of any project that depends on dxml. But unfortunately, by default, dub uses the unittest build for all dependencies if a project's unittest build is built, meaning that projects end up compiling and running unnecessary unit tests and that it only gets worse the more libraries a project depends on - even if they don't end up with linker errors like is happening with dxml.
So, to work around this unfortunate behavior, dxml now has a doTests build type to run its unit tests, and all of the unit test stuff has been versioned off with a dxml-specific version identifier. So now, not only are the linker errors gone, but projects using dxml don't have to worry about inadvertently building and running dxml's unit tests.