Unit test: properly close image file in test

On Windows it's not allowed to erase a file while it's opened, which caused
this error to surface. The file is now properly closed before the test
file is erased.

internal/manager/last_rendered/last_rendered_test.go

@@ -95,6 +95,7 @@ func TestProcessImage(t *testing.T) {
 		if !assert.NoError(t, err, "thumbnail %s should be openable", spec.filename) {
 			return
 		}
+		defer file.Close()
 
 		img, format, err := image.Decode(file)
 		if !assert.NoErrorf(t, err, "thumbnail %s should be decodable", spec.filename) {