Record all broadcast values in xtrigger return [backport]

[sadielbartholomew]

These changes close (for 7.8.x) #3275.

This PR is a direct equivalent to #3276, to backport it to 7.8.x. For reviewing, note the only difference to the counterpart PR is that xtriggers/01-suite_state.t still sits under tests/ here, rather than under flakytests/.

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required.

[sadielbartholomew]

Two test chunks have failed on Travis CI, with the results:

• CHUNK="1/4":

  Test Summary Report
  -------------------
  ./tests/authentication/05-full-control.t      (Wstat: 256 Tests: 1 Failed: 0)
    Non-zero exit status: 1
    Parse errors: Bad plan.  You planned 12 tests but ran 1.
  ./tests/authentication/01-description.t       (Wstat: 256 Tests: 1 Failed: 0)
    Non-zero exit status: 1
    Parse errors: Bad plan.  You planned 10 tests but ran 1.
  ./tests/cylc-poll/16-execution-time-limit.t   (Wstat: 0 Tests: 4 Failed: 1)
    Failed test:  4
  ./tests/cylc-review/01-title.t                (Wstat: 0 Tests: 10 Failed: 4)
    Failed tests:  4, 6, 8, 10
  ./tests/cylc-review/00-basic.t                (Wstat: 0 Tests: 64 Failed: 4)
    Failed tests:  6, 10, 28, 30
  Files=6, Tests=95, 50 wallclock secs ( 0.07 usr  0.02 sys + 33.70 cusr  5.38 csys = 39.17 CPU)
  Result: FAIL
  

• CHUNK="2/4":

  Test Summary Report
  -------------------
  ./tests/execution-time-limit/00-background.t (Wstat: 0 Tests: 5 Failed: 2)
    Failed tests:  2, 5
  ./tests/database/01-broadcast.t             (Wstat: 0 Tests: 5 Failed: 1)
    Failed test:  5
  ./tests/database/02-retry.t                 (Wstat: 0 Tests: 3 Failed: 1)
    Failed test:  3
  ./tests/database/00-simple.t                (Wstat: 0 Tests: 21 Failed: 1)
    Failed test:  6
  Files=6, Tests=58, 62 wallclock secs ( 0.05 usr  0.02 sys + 34.03 cusr  5.28 csys = 39.38 CPU)
  Result: FAIL
  

All failure cases in CHUNK 2 are known flaky tests, as moved under the flakytests/ dir in Cylc 8. For CHUNK 1, the tests under tests/authentication/ & tests/cylc-review/ are not known to be flaky, so I will look into those shortly.

Update: under an since-edited state of the leading comment in #2894, it states that tests/authentication/* do "fail randomly", hence those are also on the flaky test list, but have perhaps since been removed or renamed, hence I can't see them under Cylc 8's the flakytests/ dir.

[matthewrmshin]

These are likely failing because of the host change in Travis. Don't spend too much time on them.

[sadielbartholomew]

Don't spend too much time on them.

Sure, I wont. I just wanted to justify the failures to reassure reviewers that this is okay to merge. If you are satisfied already, that is great!

[hjoliver]

Great, thanks @sadielbartholomew 👍

[hjoliver]

The chunk 1 failures definitely can't be related to this change, so merging...