[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/version_check_command.dart

@@ -128,7 +128,7 @@ class VersionCheckCommand extends PackageLoopingCommand {
           'that intentionally has no version should be marked '
           '"publish_to: none".');
       // No remaining checks make sense, so fail immediately.
-      PackageResult.fail(<String>['No pubspec.yaml version.']);
+      return PackageResult.fail(<String>['No pubspec.yaml version.']);
     }
 
     final List<String> errors = <String>[];