Re-land "Update Zulip tests and re-enable (#441)"

[gnprice]

This re-lands 8cf7a67.

Difference from previous version: This version has the new setup step do its work only where sudo is available (in GitHub Actions), and successfully do nothing where it's unavailable (in LUCI). The step isn't needed in LUCI.

The logic lives in a setup script added today to the Zulip tree:
zulip/zulip-flutter#1300

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[gnprice]

Marked as draft because before we land this, I'd like to do a test run that confirms it does indeed work smoothly in LUCI. I'm fairly confident it will — but since the GitHub Actions / LUCI contrast already turned the tree red once this week, I'd rather make sure it won't again.

(Alternatively if flutter/flutter#162048 lands first, then this can go ahead and land and we'll get that confirmation when updating the flutter/tests pin in flutter/flutter.)

[gnprice]

OK, over in flutter/flutter#162120 I successfully ran this PR's version of customer_testing, and it passed:

+ git -C ../../bin/cache/pkg/tests checkout FETCH_HEAD
HEAD is now at f0d3df3 Use correct commit ID
+ dart --enable-asserts run_tests.dart --skip-on-fetch-failure --skip-template ../../bin/cache/pkg/tests/registry/assorted_layout_widgets.test ../../bin/cache/pkg/tests/registry/dart_rfb.test ../../bin/cache/pkg/tests/registry/flutter_cocoon.test ../../bin/cache/pkg/tests/registry/flutter_devtools_analysis.test ../../bin/cache/pkg/tests/registry/flutter_devtools.test ../../bin/cache/pkg/tests/registry/flutter_portal.test ../../bin/cache/pkg/tests/registry/flutter_rfb.test ../../bin/cache/pkg/tests/registry/macos_ui.test ../../bin/cache/pkg/tests/registry/provider.test ../../bin/cache/pkg/tests/registry/super_editor.test ../../bin/cache/pkg/tests/registry/super_sliver_list.test ../../bin/cache/pkg/tests/registry/template.test ../../bin/cache/pkg/tests/registry/zulip.test

Processing ./../../bin/cache/pkg/tests/registry/assorted_layout_widgets.test...
Tests finished in 18.00 seconds per iteration.
[…]
Processing ./../../bin/cache/pkg/tests/registry/super_editor.test...
Tests finished in 220.00 seconds per iteration.
Processing ./../../bin/cache/pkg/tests/registry/super_sliver_list.test...
Tests finished in 38.00 seconds per iteration.
Processing ./../../bin/cache/pkg/tests/registry/zulip.test...
Tests finished in 99.00 seconds per iteration.
19 tests all passed!
ELAPSED TIME: 34min 8.683s for ../../ci.sh  in /b/s/w/ir/x/w/flutter/dev/customer_testing
▌21:06:48▐ Test successful.

LUCI logs:
https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20customer_testing/91700/overview
https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8724890549871873921/+/u/run_test.dart_for_customer_testing_shard_and_subshard_None/stdout

Meanwhile it also passed here in the GitHub Actions-based CI for this repo. So I think everything looks clear for this to merge.