Multiple observed warnings combined in display

[MichaelChirico]

As seen in #6116. Before/after:

test(1, {warning(1); warning(2); TRUE})
## BEFORE
Test 1 produced 2 warnings but expected 0
Expected: 
Observed: 1
 Test 1 produced 2 warnings but expected 0
Expected: 
Observed: 2

## AFTER
Test 1 produced 2 warnings but expected 0
Expected: 
Observed: 1
Observed: 2

[github-actions]

Generated via commit 17f77e9

Download link for the artifact containing the test results: ↓ atime-results.zip

Time taken to finish the standard R installation steps: 11 minutes and 43 seconds

Time taken to run atime::atime_pkg on the tests: 3 minutes and 20 seconds

[tdhock]

great thanks