[flutter_plugin_tool] Add more failure test coverage (#4132)

Many commands had insufficient failure testing. This adds new tests that
ensure that for every Process call, at least one test fails if a failure
from that process were ignored (with the exception of calls in the
`publish` command, which has a custom process mocking system, so was out
of scope here; it already has more coverage than most tests did though.)

For a few existing failure tests, adds output checks to ensure that they
are testing for the *right* failures.

Other changes:
- Adds convenience constructors to MockProcess for the common cases of a
  mock process that just exits with a 0 or 1 status, to reduce test
  verbosity.
- Fixes a few bugs that were found by the new tests.
- Minor test cleanup, especially cases where a mock process was being
  set up just to make all calls succeed, which is the default as of
  recent changes.

script/tool/test/publish_check_command_test.dart

@@ -46,10 +46,10 @@ void main() {
           createFakePlugin('plugin_tools_test_package_b', packagesDir);
 
       processRunner.processesToReturn.add(
-        MockProcess()..exitCodeCompleter.complete(0),
+        MockProcess.succeeding(),
       );
       processRunner.processesToReturn.add(
-        MockProcess()..exitCodeCompleter.complete(0),
+        MockProcess.succeeding(),
       );
       await runCapturingPrint(runner, <String>['publish-check']);
 
@@ -70,10 +70,9 @@ void main() {
     test('fail on negative test', () async {
       createFakePlugin('plugin_tools_test_package_a', packagesDir);
 
-      final MockProcess process = MockProcess();
+      final MockProcess process = MockProcess.failing();
       process.stdoutController.close(); // ignore: unawaited_futures
       process.stderrController.close(); // ignore: unawaited_futures
-      process.exitCodeCompleter.complete(1);
 
       processRunner.processesToReturn.add(process);
 
@@ -100,13 +99,11 @@ void main() {
       const String preReleaseOutput = 'Package has 1 warning.'
           'Packages with an SDK constraint on a pre-release of the Dart SDK should themselves be published as a pre-release version.';
 
-      final MockProcess process = MockProcess();
+      final MockProcess process = MockProcess.failing();
       process.stdoutController.add(preReleaseOutput.codeUnits);
       process.stdoutController.close(); // ignore: unawaited_futures
       process.stderrController.close(); // ignore: unawaited_futures
 
-      process.exitCodeCompleter.complete(1);
-
       processRunner.processesToReturn.add(process);
 
       expect(
@@ -121,13 +118,11 @@ void main() {
       const String preReleaseOutput = 'Package has 1 warning.'
           'Packages with an SDK constraint on a pre-release of the Dart SDK should themselves be published as a pre-release version.';
 
-      final MockProcess process = MockProcess();
+      final MockProcess process = MockProcess.failing();
       process.stdoutController.add(preReleaseOutput.codeUnits);
       process.stdoutController.close(); // ignore: unawaited_futures
       process.stderrController.close(); // ignore: unawaited_futures
 
-      process.exitCodeCompleter.complete(1);
-
       processRunner.processesToReturn.add(process);
 
       expect(runCapturingPrint(runner, <String>['publish-check']),
@@ -139,13 +134,11 @@ void main() {
 
       const String publishOutput = 'Package has 0 warnings.';
 
-      final MockProcess process = MockProcess();
+      final MockProcess process = MockProcess.succeeding();
       process.stderrController.add(publishOutput.codeUnits);
       process.stdoutController.close(); // ignore: unawaited_futures
       process.stderrController.close(); // ignore: unawaited_futures
 
-      process.exitCodeCompleter.complete(0);
-
       processRunner.processesToReturn.add(process);
 
       final List<String> output =
@@ -195,7 +188,7 @@ void main() {
       createFakePlugin('no_publish_b', packagesDir, version: '0.2.0');
 
       processRunner.processesToReturn.add(
-        MockProcess()..exitCodeCompleter.complete(0),
+        MockProcess.succeeding(),
       );
       final List<String> output = await runCapturingPrint(
           runner, <String>['publish-check', '--machine']);
@@ -261,7 +254,7 @@ void main() {
       createFakePlugin('no_publish_b', packagesDir, version: '0.2.0');
 
       processRunner.processesToReturn.add(
-        MockProcess()..exitCodeCompleter.complete(0),
+        MockProcess.succeeding(),
       );
 
       final List<String> output = await runCapturingPrint(
@@ -334,7 +327,7 @@ void main() {
       await plugin1Dir.childFile('pubspec.yaml').writeAsString('bad-yaml');
 
       processRunner.processesToReturn.add(
-        MockProcess()..exitCodeCompleter.complete(0),
+        MockProcess.succeeding(),
       );
 
       bool hasError = false;