Add MockFileSource and MockView for testing the ResourceLoader

- MockFileSource
  Will load pre-defined resources from the filesystem. It supports
  passing a string, that when it matches an URL, will simulate a failure.

- MockView
  It is just a no-op to not get in the way when testing
  resource loading.

Few more additions:

- style.json
  Specially crafted style that will look always for the same tile,
  glyph range, etc at the "test/fixtures/resources/" path. No matter
  where you look (or at what zoom), you always get the same stuff.

- vector.pbf
  This taken from a part of Taipei that will request several glyph
  ranges.

test/fixtures/resources/style.json

@@ -0,0 +1,31 @@
+{
+  "version": 1,
+  "name": "Test",
+  "sources": {
+    "compositedsource": {
+      "url": "test/fixtures/resources/source.json",
+      "type": "vector"
+    }
+  },
+  "sprite": "test/fixtures/resources/sprite",
+  "glyphs": "test/fixtures/resources/glyphs.pbf",
+  "layers": [{
+    "id": "road",
+    "type": "symbol",
+    "source": "compositedsource",
+    "source-layer": "road_label",
+    "layout": {
+      "text-font": "Open Sans Regular, Arial Unicode MS Regular",
+      "text-field": "{name_en}"
+    }
+  }, {
+    "id": "poi",
+    "type": "symbol",
+    "source": "compositedsource",
+    "source-layer": "poi_label",
+    "layout": {
+      "text-font": "Open Sans Regular, Arial Unicode MS Regular",
+      "icon-image": "{maki}_icon"
+    }
+  }]
+}

test/resources/mock_file_source.cpp

@@ -0,0 +1,47 @@
+#include "mock_file_source.hpp"
+
+#include <mbgl/storage/request.hpp>
+#include <mbgl/util/io.hpp>
+#include <mbgl/util/thread.hpp>
+
+namespace mbgl {
+
+class MockFileSource::Impl {
+public:
+    Impl(uv_loop_t*, const std::string& matchFail) : matchFail_(matchFail) {}
+
+    void handleRequest(Request* req) const;
+
+private:
+    std::string matchFail_;
+};
+
+void MockFileSource::Impl::handleRequest(Request* req) const {
+    const Resource& resource = req->resource;
+
+    std::shared_ptr<Response> response = std::make_shared<Response>();
+    if (matchFail_.empty() || resource.url.find(matchFail_) == std::string::npos) {
+        response->status = Response::Status::Successful;
+        response->data = util::read_file(resource.url.c_str());
+    } else {
+        response->message = "Failed by the test case";
+    }
+
+    req->notify(response);
+}
+
+MockFileSource::MockFileSource(const std::string& matchFail)
+    : thread_(util::make_unique<util::Thread<Impl>>("FileSource", util::ThreadPriority::Low, matchFail)) {
+}
+
+Request* MockFileSource::request(const Resource& resource, uv_loop_t* loop, Callback callback) {
+    Request* req = new Request(resource, loop, std::move(callback));
+    thread_->invoke(&Impl::handleRequest, req);
+
+    return req;
+}
+
+void MockFileSource::cancel(Request*) {
+}
+
+}

test/resources/mock_file_source.hpp

@@ -0,0 +1,33 @@
+#ifndef TEST_RESOURCES_MOCK_FILE_SOURCE
+#define TEST_RESOURCES_MOCK_FILE_SOURCE
+
+#include <mbgl/storage/file_source.hpp>
+
+#include <string>
+
+namespace mbgl {
+
+namespace util {
+template <typename T> class Thread;
+}
+
+// This mock FileSource will read data from the disk and will fail
+// the request if the URL matches the 'matchFail' string.
+class MockFileSource : public FileSource {
+public:
+    class Impl;
+
+    MockFileSource(const std::string& matchFail);
+    ~MockFileSource() override = default;
+
+    // FileSource implementation.
+    Request* request(const Resource&, uv_loop_t*, Callback) override;
+    void cancel(Request*) override;
+
+private:
+    const std::unique_ptr<util::Thread<Impl>> thread_;
+};
+
+}
+
+#endif

test/resources/mock_view.hpp

@@ -0,0 +1,21 @@
+#ifndef TEST_RESOURCES_MOCK_VIEW
+#define TEST_RESOURCES_MOCK_VIEW
+
+#include <mbgl/map/view.hpp>
+
+namespace mbgl {
+
+class MockView : public View {
+public:
+    MockView() = default;
+
+    // View implementation.
+    void activate() override {};
+    void deactivate() override {};
+    void notify() override {};
+    void invalidate(std::function<void()>) override {};
+};
+
+}
+
+#endif

test/test.gypi

@@ -59,6 +59,10 @@
         'miscellaneous/variant.cpp',
         'miscellaneous/worker.cpp',
 
+        'resources/mock_file_source.cpp',
+        'resources/mock_file_source.hpp',
+        'resources/mock_view.hpp',
+
         'storage/storage.hpp',
         'storage/storage.cpp',
         'storage/cache_response.cpp',