test: Ignore issue-19501 pretty for now

I don't have enough time to investigate this thoroughly, so for now let's get
the queue moving by ignoring this test.

cc #19501
auto merge of #19502 : alexcrichton/rust/issue-19501, r=sfackler

I don't have enough time to investigate this thoroughly, so for now let's get
the queue moving by ignoring this test.

cc #19501