test: Ignore a deadlocking test on OSX

This test apparently keeps making no progress and timing out builds on the OSX
builder, so this commit is switching the test to be ignored.

cc #16872
auto merge of #16873 : alexcrichton/rust/ignore-deadlocks, r=sfackler

This test apparently keeps making no progress and timing out builds on the OSX
builder, so this commit is switching the test to be ignored.

cc #16872