test_runner: display diagnostic messages in dot reporter

When using the dot reporter with coverage enabled, coverage failures
would silently exit with an error code but show no output explaining
the failure. This change collects and displays diagnostic messages
(including coverage warnings) after the test dots, similar to how
failed tests are displayed.

Fixes: #60884
Signed-off-by: armorbreak001 <contact@agentvote.cc>

Author: armorbreak001

lib/internal/test_runner/reporter/dot.js

@@ -10,6 +10,7 @@ module.exports = async function* dot(source) {
   let count = 0;
   let columns = getLineLength();
   const failedTests = [];
+  const diagnostics = [];
   for await (const { type, data } of source) {
     if (type === 'test:pass') {
       yield `${colors.green}.${colors.reset}`;
@@ -18,6 +19,9 @@ module.exports = async function* dot(source) {
       yield `${colors.red}X${colors.reset}`;
       ArrayPrototypePush(failedTests, data);
     }
+    if (type === 'test:diagnostic') {
+      ArrayPrototypePush(diagnostics, data);
+    }
     if ((type === 'test:fail' || type === 'test:pass') && ++count === columns) {
       yield '\n';
 
@@ -33,6 +37,12 @@ module.exports = async function* dot(source) {
       yield formatTestReport('test:fail', test);
     }
   }
+  if (diagnostics.length > 0) {
+    yield `\n${colors.yellow}Diagnostics:${colors.white}\n\n`;
+    for (const diag of diagnostics) {
+      yield `${colors.yellow}! ${diag.message}${colors.reset}\n`;
+    }
+  }
 };
 
 function getLineLength() {