Tests: introduce dedicated tests for text localization

So far, the methods related to text localization - `PHPMailer::setLanguage()`, `PHPMailer::getTranslations()` and `PHPMailer::lang()` - did not have dedicated tests.

The test file this commit introduces, tests all aspects of these methods, including the changes introduced in response to 2418 and 2419, as well as documents the current behaviour of the methods for specific, outlier situations.

test/Fixtures/LocalizationTest/phpmailer.lang-fr.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test overruling an existing translation
+ * file with a custom one by passing in a `$langPath` parameter.
+ */
+
+$PHPMAILER_LANG['empty_message'] = "Custom path test success (fr)";

test/Fixtures/LocalizationTest/phpmailer.lang-nl.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test overruling an existing translation
+ * file with a custom one by passing in a `$langPath` parameter.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'Custom path test success (nl)';

test/Fixtures/LocalizationTest/phpmailer.lang-xa_scri_cc.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test the fall-back logic.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'XA Lang-script-country file found';

test/Fixtures/LocalizationTest/phpmailer.lang-xb_scri.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test the fall-back logic.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'XB Lang-script file found';

test/Fixtures/LocalizationTest/phpmailer.lang-xc_cc.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test the fall-back logic.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'XC Lang-country file found';

test/Fixtures/LocalizationTest/phpmailer.lang-xd_cc.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test the fall-back logic.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'XD Lang-country file found';

test/Fixtures/LocalizationTest/phpmailer.lang-xd_scri.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test the fall-back logic.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'XD Lang-script file found';

test/Fixtures/LocalizationTest/phpmailer.lang-xe.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test the fall-back logic.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'XE Lang file found';

test/Fixtures/LocalizationTest/phpmailer.lang-xx.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest`.
+ */
+
+$PHPMAILER_LANG['empty_message'] = 'This file should not be loaded as the path is passed incorrectly';

test/Fixtures/LocalizationTest/phpmailer.lang-yy.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test that arbitrary code in translation files is disregarded.
+ */
+
+$composer = file_get_contents(__DIR__ . '/../../../composer.json');
+
+echo $composer;
+
+$PHPMAILER_LANG['extension_missing'] = 'Confirming that test fixture was loaded correctly (yy).';
+$PHPMAILER_LANG['empty_message']     = $composer;
+$PHPMAILER_LANG['encoding']          = `ls -l`;
+$PHPMAILER_LANG['execute']           = exec('some harmful command');
+$PHPMAILER_LANG['signing']           = "Double quoted but not interpolated $composer";

test/Fixtures/LocalizationTest/phpmailer.lang-zz.php

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+<?php
+
+/**
+ * Test fixture.
+ *
+ * Used in the `PHPMailer\LocalizationTest` to test that language strings not
+ * set via a fixed, known group of array index keys are disregarded.
+ */
+
+$PHPMAILER_LANG['extension_missing'] = 'Confirming that test fixture was loaded correctly (zz).';
+
+// Keys not in the original array.
+$PHPMAILER_LANG['unknown']           = 'Unknown text.';
+$PHPMAILER_LANG['invalid']           = 'Invalid text.';
+
+// Keys which exist in the original array, but use the wrong letter case or space instead of underscore.
+$PHPMAILER_LANG['Authenticate']      = 'Overruled text, index not same case';
+$PHPMAILER_LANG['CONNECT_HOST']      = 'Overruled text, index not same case';
+$PHPMAILER_LANG['Data_Not_Accepted'] = 'Overruled text, index not same case';
+$PHPMAILER_LANG['empty message']     = 'Overruled text, index not same case';