user32/tests: Disable automatic shell restart during SetShellWindow tests.

[Hans Leidekker]

On Tue, 2016-12-20 at 05:31 -0600, Marvin wrote:
> === w864 (32 bit win) ===
> win.c:2305: Test failed: expected !100
> win.c:2305: Test failed: expected !100
> 
> === w864 (64 bit win) ===
> win.c:2305: Test failed: expected !100
> win.c:2305: Test failed: expected !100

This failure is also seen with a whitespace patch:
https://testbot.winehq.org/JobDetails.pl?Key=27439