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/health_test.go

@@ -51,7 +51,7 @@ func TestReturns503IfThereAreErrorChecks(t *testing.T) {
 // the web application when things aren't so healthy.
 func TestHealthHandler(t *testing.T) {
 	// clear out existing checks.
-	registeredChecks = make(map[string]Checker)
+	DefaultRegistry = NewRegistry()
 
 	// protect an http server
 	handler := http.Handler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {