Add test for FontContext/FontGroup functionality

Unfortunately, this required quite a bit of changes to the non-test
code. That's because FontContext depends on a FontCacheThread, which in
turn depends on a CoreResourceThread and therefore lots of other data
structures.

It seemed like it would be very difficult to instantiate a FontContext
as it was, and even if we could it seems like overkill to have all these
data structures present for a relatively focused test.

Therefore, I created a FontSource trait which represents the interface
which FontContext uses to talk to FontCacheThread. FontCacheThread then
implements FontSource. Then, in the test, we can create a dummy
implementation of FontSource rather than using FontCacheThread.

This actually has the advantage that we can make our dummy
implementation behave in certain specific way which are useful for
testing, for example it can count the number of times
find_font_template() is called, which helps us verify that
caching/lazy-loading is working as intended.

components/layout/context.rs

@@ -27,10 +27,12 @@ use std::thread;
 use style::context::RegisteredSpeculativePainter;
 use style::context::SharedStyleContext;
 
-thread_local!(static FONT_CONTEXT_KEY: RefCell<Option<FontContext>> = RefCell::new(None));
+pub type LayoutFontContext = FontContext<FontCacheThread>;
+
+thread_local!(static FONT_CONTEXT_KEY: RefCell<Option<LayoutFontContext>> = RefCell::new(None));
 
 pub fn with_thread_local_font_context<F, R>(layout_context: &LayoutContext, f: F) -> R
-    where F: FnOnce(&mut FontContext) -> R
+    where F: FnOnce(&mut LayoutFontContext) -> R
 {
     FONT_CONTEXT_KEY.with(|k| {
         let mut font_context = k.borrow_mut();