nixos/tests/chromium: Wait 10s after new window

This is a very very very ugly workaround and it's because Chromium seems
to eat keystroke for a few seconds after a new window is created.

I haven't found a better solution yet, so let's at least unbreak the
test until we come up with a better way.

Thanks to @vcunat for bringing this to my attention and also doing the
initial bisect.

The change that brought up this problem was 2b29e401,
which updated Chromium from version 65.0.3325.181 to version
66.0.3359.117. Unfortunately the upstream changelog[1] is way too large
to actually guess what the breaking change is.

[1]: https://chromium.googlesource.com/chromium/src/+log/65.0.3325.181..66.0.3359.117?pretty=fuller&n=10000



Signed-off-by: aszlig <aszlig@nix.build>
Cc: @bendlas, @vcunat