Fix incorrectly formatted messages used in test cases.

PoFileParser returns different format actually.

tests/TestCase/I18n/I18nTest.php

@@ -326,6 +326,12 @@ class I18nTest extends TestCase
             $package->setMessages([
                 'letter' => [
                     '_context' => [
+                        'character' => 'The letter {0}',
+                        'communication' => 'The letters {0} and {1}',
+                    ]
+                ],
+                'letters' => [
+                    '_context' => [
                         'character' => [
                             'The letter {0}',
                             'The letters {0} and {1}'
@@ -391,6 +397,12 @@ class I18nTest extends TestCase
             $package->setMessages([
                 'letter' => [
                     '_context' => [
+                        'character' => 'The letter {0}',
+                        'communication' => 'The letters {0} and {1}',
+                    ]
+                ],
+                'letters' => [
+                    '_context' => [
                         'character' => [
                             'The letter {0}',
                             'The letters {0} and {1}'