Fix up and run unit tests.

This adds the subpackages to `./mach test-unit`.
auto merge of #3566 : metajack/servo/fixup-unit-tests, r=mbrubeck

This adds the subpackages to `./mach test-unit`.
Add Android build config to mach/servobuild
auto merge of #3568 : mbrubeck/servo/mach-android, r=metajack

This allows setting Android toolchain paths in `.servobuild`, and also adds a `--android` option that sets the correct default target and causes `mach build` to also build the APK.
Account for RenderLayer position when optimizing display list

The page_rect passed to DisplayListOptimizer is relative to the
RenderLayer origin, but the display list components are relative to the
page origin. Before passing the page rect to the display list, we
translate it by the RenderLayer position.
auto merge of #3567 : mrobinson/servo/optimizer, r=zwarich

The page_rect passed to DisplayListOptimizer is relative to the
RenderLayer origin, but the display list components are relative to the
page origin. Before passing the page rect to the display list, we
translate it by the RenderLayer position.
Replace Cell<Option<JS<T>>> with MutNullableJS<T>

https://github.com/servo/servo/issues/3564
auto merge of #3569 : andrewguertin/servo/mutnullablejs, r=Manishearth

https://github.com/servo/servo/issues/3564