Include stdout/stderr in failure messages for devicelab's tests (#18990)

* Include stdout/stderr in failure messages

Sometimes some of these tests unexpectedly fail with a non-zero exit code. This ensures stdout/stderr is included in the test failure message when this happens so that we can track down the issue.

* Remove redundant info about exit code

* Remove unnecessary indenting

The [markers] make it fairly clear so this just makes the test code noisy.

dev/devicelab/test/run_test.dart

@@ -13,7 +13,7 @@ void main() {
   const ProcessManager processManager = const LocalProcessManager();
 
   group('run.dart script', () {
-    Future<int> runScript(List<String> testNames) async {
+    Future<ProcessResult> runScript(List<String> testNames) async {
       final List<String> options = <String>['bin/run.dart'];
       for (String testName in testNames) {
         options..addAll(<String>['-t', testName]);
@@ -22,39 +22,45 @@ void main() {
       final ProcessResult scriptProcess = processManager.runSync(
         <String>[dart]..addAll(options)
       );
-      return scriptProcess.exitCode;
+      return scriptProcess;
+    }
+
+    Future<void> expectScriptResult(List<String> testNames, int expectedExitCode) async {
+      final ProcessResult result = await runScript(testNames);
+      expect(result.exitCode, expectedExitCode,
+          reason: '[ stderr from test process ]\n\n${result.stderr}\n\n[ end of stderr ]'
+          '\n\n[ stdout from test process ]\n\n${result.stdout}\n\n[ end of stdout ]');
     }
 
     test('exits with code 0 when succeeds', () async {
-      expect(await runScript(<String>['smoke_test_success']), 0);
+      await expectScriptResult(<String>['smoke_test_success'], 0);
     });
 
     test('accepts file paths', () async {
-      expect(await runScript(<String>['bin/tasks/smoke_test_success.dart']), 0);
+      await expectScriptResult(<String>['bin/tasks/smoke_test_success.dart'], 0);
     });
 
     test('rejects invalid file paths', () async {
-      expect(await runScript(<String>['lib/framework/adb.dart']), 1);
+      await expectScriptResult(<String>['lib/framework/adb.dart'], 1);
     });
 
     test('exits with code 1 when task throws', () async {
-      expect(await runScript(<String>['smoke_test_throws']), 1);
+      await expectScriptResult(<String>['smoke_test_throws'], 1);
     });
 
     test('exits with code 1 when fails', () async {
-      expect(await runScript(<String>['smoke_test_failure']), 1);
+      await expectScriptResult(<String>['smoke_test_failure'], 1);
     });
 
     test('exits with code 1 when fails to connect', () async {
-      expect(await runScript(<String>['smoke_test_setup_failure']), 1);
+      await expectScriptResult(<String>['smoke_test_setup_failure'], 1);
     }, skip: true); // https://github.com/flutter/flutter/issues/5901
 
     test('exits with code 1 when results are mixed', () async {
-      expect(
-        await runScript(<String>[
+      await expectScriptResult(<String>[
           'smoke_test_failure',
           'smoke_test_success',
-        ]),
+        ],
         1,
       );
     });