Create success job for automerge

[hugovk]

For #3.

Changes proposed in this pull request:

• This creates a no-op "test successful" job, that only runs when the test matrix is successful
• Then automerge can have this success job as a required status check in the branch protection rules
• Means we don't need to tediously add/remove individual matrix entries as they change

[codecov-commenter]

Codecov Report

Merging #5 (41bafd4) into main (368fd4f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main       #5   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files           9        9           
  Lines         635      635           
=======================================
  Hits          633      633           
  Misses          2        2           

Flag	Coverage Δ
GHA_Ubuntu	98.42% <ø> (ø)
GHA_Windows	98.11% <ø> (ø)
GHA_macOS	98.42% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 368fd4f...41bafd4. Read the comment docs.