Fix test suite on macos

[dbuenzli]

For some reason this line:

https://github.com/dbuenzli/tsdl/blob/9e8403065fa53d46adefd7d196c659e058ef7099/test/test.ml#L630

Results in a:

-[_MTLCommandBuffer presentDrawable:]:602: failed assertion `drawable must not be nil.'

on macOS

[dbuenzli]

Something about a similar error upstream https://github.com/libsdl-org/SDL/issues/4863

[dbuenzli]

For some reason this line:

Then don't do that.

According the SDL3 docs there's no need to call the function when rendering to texture, which is what the test is doing.