DrawManager: Fixed memory leak in test cases

Memory leak is introduced as test cases reinitializes the GPU stack.
Added a call to GPU_backend_exit to fix this.

In GPU_backend_exit the GPU backend was destroyed but the pointer wasn't
reset for reuse. This patch also clears the pointer to be reused.

source/blender/draw/tests/shaders_test.cc

@@ -37,6 +37,7 @@ class DrawTest : public ::testing::Test {
   void TearDown() override
   {
     GPU_exit();
+    GPU_backend_exit();
     GPU_context_discard(context);
     GHOST_DisposeOpenGLContext(ghost_system, ghost_context);
     GHOST_DisposeSystem(ghost_system);

source/blender/gpu/intern/gpu_context.cc

@@ -185,6 +185,7 @@ void GPU_backend_exit(void)
   /* TODO assert no resource left. Currently UI textures are still not freed in their context
    * correctly. */
   delete g_backend;
+  g_backend = NULL;
 }
 
 GPUBackend *GPUBackend::get(void)