Expose a Registry type in health package, so unit tests can stay isolated from each other

Update docs.

Change health_test.go tests to create their own registries and register
the checks there. The tests now call CheckStatus directly instead of
polling the HTTP handler, which returns results from the default
registry.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

health/doc.go

@@ -39,7 +39,7 @@
 //
 // The recommended way of registering checks is using a periodic Check.
 // PeriodicChecks run on a certain schedule and asynchronously update the
-// status of the check. This allows "CheckStatus()" to return without blocking
+// status of the check. This allows CheckStatus to return without blocking
 // on an expensive check.
 //
 // A trivial example of a check that runs every 5 seconds and shuts down our