test_runner: display coverage errors in dot reporter

[elephant-y]

When using the dot reporter with --experimental-test-coverage and
coverage thresholds, if a threshold is not met, the dot reporter
outputs only dots (.) with no error message. The process exits with
code 1 but gives no indication of what went wrong.

This happens because the dot reporter does not handle
test:diagnostic events with error level, which are emitted when
coverage thresholds are not satisfied.

Changes

• Added handling for test:diagnostic events with level === 'error'
• Collected error messages are displayed after the dot output,
  similar to how failed tests are displayed

Before

$ node --test --experimental-test-coverage --test-reporter=dot \
  --test-coverage-lines=100 test.js
.

$ echo $?
1           # failed but no error shown

After

$ node --test --experimental-test-coverage --test-reporter=dot \
  --test-coverage-lines=100 test.js
.

Diagnostic errors:

Error: 85.00% line coverage does not meet threshold of 100.00%.
$ echo $?
1

Fixes: #60884

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner