Move MockFileSource to fixtures

It is going to be used by another test that needs to emulate
asynchronous requests.

test/style/pending_resources.cpp

@@ -1,6 +1,6 @@
 #include "../fixtures/fixture_log_observer.hpp"
+#include "../fixtures/mock_file_source.hpp"
 #include "../fixtures/util.hpp"
-#include "mock_file_source.hpp"
 
 #include <mbgl/map/map.hpp>
 #include <mbgl/map/still_image.hpp>

test/style/resource_loading.cpp

@@ -1,7 +1,7 @@
 #include "../fixtures/fixture_log_observer.hpp"
 #include "../fixtures/util.hpp"
+#include "../fixtures/mock_file_source.hpp"
 #include "../fixtures/mock_view.hpp"
-#include "mock_file_source.hpp"
 
 #include <mbgl/map/map_data.hpp>
 #include <mbgl/map/transform.hpp>

test/style/sprite.cpp

@@ -1,6 +1,6 @@
 #include "../fixtures/fixture_log_observer.hpp"
+#include "../fixtures/mock_file_source.hpp"
 #include "../fixtures/util.hpp"
-#include "mock_file_source.hpp"
 
 #include <mbgl/map/sprite.hpp>
 #include <mbgl/util/run_loop.hpp>

test/test.gypi

@@ -30,6 +30,9 @@
       ],
       'sources': [
         'fixtures/main.cpp',
+        'fixtures/mock_file_source.cpp',
+        'fixtures/mock_file_source.hpp',
+        'fixtures/mock_view.hpp',
         'fixtures/util.hpp',
         'fixtures/util.cpp',
         'fixtures/fixture_log_observer.hpp',
@@ -83,9 +86,6 @@
         'storage/http_other_loop.cpp',
         'storage/http_reading.cpp',
 
-        'style/mock_file_source.cpp',
-        'style/mock_file_source.hpp',
-        'style/mock_view.hpp',
         'style/pending_resources.cpp',
         'style/resource_loading.cpp',
         'style/sprite.cpp',