explorerframe/tests: Use EqualRect() instead of open coding it.

[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=24131

Your paranoid android.


=== w7pro64 (32 bit nstc) ===
nstc.c:1941: Test failed: Got event 17, count 1

=== w7pro64 (64 bit nstc) ===
nstc.c:1951: Test failed: Got event 17, count 1