shell32/tests: Only look for Explorer windows for the progman tests.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=20640

Your paranoid android.


=== w2003std (32 bit progman_dde) ===
progman_dde.c:383: Test failed: Window "C:\Documents and Settings\All Users\Start Menu\Programs\Group1" was not created in 45 seconds - assumed failure.  [S_G:12]