tests/screentest: omit mobile expanded README test

Comment out the test for expanded README on small screens.

It fails about half the time in CI, making it annoying to
deploy and submit CLs.

Maybe more effort could deflake it, but I'm not optimistic. I spent
a while throwing everything I could at it and it's still misbehaving.
It would require someone more familiar than I with the intricacies
of HTML page loading and headless chrome to make progress.

It is only for mobile, not our main concern, and it is only one test.

Change-Id: Icd9c2bd3f3c99ee0efaf8a617f9b2b31a0c8dde6
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/646855
kokoro-CI: kokoro <noreply+kokoro@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

tests/screentest/testcases.txt

@@ -47,7 +47,8 @@ eval {{$scrollTop}}
 eval {{$hideElements}}
 sleep 3s
 capture viewport
-capture viewport 540x1080
+# This fails about half the time in CI.
+# capture viewport 540x1080
 
 test x/net directories expanded
 path /golang.org/x/net@v0.31.0