coverage: exclude report-timings and set 90% as the lower limit

[oliver-sanders]

• Exclude the deprecated report-timings command from code coverage.
• Don't allow coverage to dip below 90%.

Check List

• 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).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Linter detected a stale link: http://code.activestate.com/recipes/66012-fork-a-daemon-process-on-unix/

Still listed by Google search, but the site does appear to down at the moment.

Anyway, nothing to do with this branch.