NXTTest: expose SwapBuffers to test to make it easy to RenderDoc

RenderDoc uses presents as the points to start and end a capture, so to
debug test we have to do the following:

TEST_F(...)
    while(true) {
    	// All the test code
    	SwapBuffers();
    }
}

src/tests/NXTTest.cpp

@@ -188,6 +188,9 @@ void NXTTest::AddTextureExpectation(const char* file, int line, const nxt::Textu
     deferredExpectations.push_back(deferred);
 }
 
+void NXTTest::SwapBuffers() {
+    binding->SwapBuffers();
+}
 
 NXTTest::ReadbackReservation NXTTest::ReserveReadback(uint32_t readbackSize) {
     // For now create a new MapRead buffer for each readback

src/tests/NXTTest.h

@@ -72,6 +72,8 @@ class NXTTest : public ::testing::TestWithParam<BackendType> {
         void AddBufferExpectation(const char* file, int line, const nxt::Buffer& buffer, uint32_t offset, uint32_t size, detail::Expectation* expectation);
         void AddTextureExpectation(const char* file, int line, const nxt::Texture& texture, uint32_t x, uint32_t y, uint32_t width, uint32_t height, uint32_t pixelSize, detail::Expectation* expectation);
 
+        void SwapBuffers();
+
     private:
         // MapRead buffers used to get data for the expectations
         struct ReadbackSlot {