[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/lib/src/firebase_test_lab_command.dart

@@ -135,13 +135,13 @@ class FirebaseTestLabCommand extends PackageLoopingCommand {
 
     // Ensures that gradle wrapper exists
     if (!await _ensureGradleWrapperExists(androidDirectory)) {
-      PackageResult.fail(<String>['Unable to build example apk']);
+      return PackageResult.fail(<String>['Unable to build example apk']);
     }
 
     await _configureFirebaseProject();
 
     if (!await _runGradle(androidDirectory, 'app:assembleAndroidTest')) {
-      PackageResult.fail(<String>['Unable to assemble androidTest']);
+      return PackageResult.fail(<String>['Unable to assemble androidTest']);
     }
 
     final List<String> errors = <String>[];
@@ -236,7 +236,7 @@ class FirebaseTestLabCommand extends PackageLoopingCommand {
         : null;
 
     final int exitCode = await processRunner.runAndStream(
-        p.join(directory.path, _gradleWrapper),
+        directory.childFile(_gradleWrapper).path,
         <String>[
           target,
           '-Pverbose=true',