test/screentest: disable flaky test

For golang/go#51020.

Change-Id: If7424a89d323db7b2aecf4b2eec087159b979139
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/383198
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/tests/screentest/testcases.txt b/tests/screentest/testcases.txt
index 7217a1b..8ac4986 100644
--- a/tests/screentest/testcases.txt
+++ b/tests/screentest/testcases.txt
@@ -42,12 +42,13 @@
 capture fullscreen
 capture fullscreen 540x1080
 
-test empty jump to modal
-pathname /golang.org/x/tools@v0.1.1
-wait {{$ready}}
-click [data-test-id="jump-to-button"]
-eval {{$hideImportedby}}
-capture viewport
+# TODO: https://go.dev/issue/51020
+# test empty jump to modal
+# pathname /golang.org/x/tools@v0.1.1
+# wait {{$ready}}
+# click [data-test-id="jump-to-button"]
+# eval {{$hideImportedby}}
+# capture viewport
 
 test standard library package
 pathname /errors@go1.16.3