The tests added by #20972, which should have fixed it.
Example run: https://travis-ci.org/ampproject/amphtml/jobs/503126033
Local Safari test passing:

Additional context: When run by themselves on SauceLabs they pass:

@zombifier
@zombifier we recently fixed a couple of issues (#23519 #23512 #23543) that might have caused those flakiness. I also added some extra request-bank error message to ease the debugging.
That said there is a good chance that those tests are easier to deal with now. There are 4 remaining tests need to be unskipped. Would you take another look?
Made #23605 to try reenabling the test and see what happens, but they weirdly enough still fail on Travis. Local testing with Firefox and Chrome works fine.
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Most helpful comment
@zombifier we recently fixed a couple of issues (#23519 #23512 #23543) that might have caused those flakiness. I also added some extra request-bank error message to ease the debugging.
That said there is a good chance that those tests are easier to deal with now. There are 4 remaining tests need to be unskipped. Would you take another look?