AddEmbeddedImageTest: split off failure test

The "failure" case when a non-existent file was being passed, wasn't actually being tested at all as no assertion was used.

This commit:
* Moves the particular failure test case to a separate test method with a data provider (to allow for more failure test cases to be added).
* Uses an assertion on the call to `addEmbeddedImage()` to actually test that the method return a failure state.
* Verifies that no attachment for an inline image was added by adding a second assertion with a call to `PHPMailer::inlineImageExists()`.

test/PHPMailer/AddEmbeddedImageTest.php

@@ -53,10 +53,42 @@ final class AddEmbeddedImageTest extends PreSendTestCase
         $this->Mail->clearAttachments();
 
         //For code coverage
-        $this->Mail->addEmbeddedImage('thisfiledoesntexist', 'xyz'); //Non-existent file
         $this->Mail->addEmbeddedImage(__FILE__, '123'); //Missing name
     }
 
+    /**
+     * Test that embedding an image fails in select use cases.
+     *
+     * @dataProvider dataFailToAttach
+     *
+     * @param string $path     Path to the attachment.
+     * @param string $cid      Content ID for the attachment.
+     * @param string $name     Optional. Attachment name to use.
+     * @param string $encoding Optional. File encoding to pass.
+     */
+    public function testFailToAttach($path, $cid, $name = '', $encoding = PHPMailer::ENCODING_BASE64)
+    {
+        $result = $this->Mail->addEmbeddedImage($path, $cid, $name, $encoding);
+        self::assertFalse($result, 'Image did not fail to attach');
+
+        self::assertFalse($this->Mail->inlineImageExists(), 'Inline image present in attachments array');
+    }
+
+    /**
+     * Data provider.
+     *
+     * @return array
+     */
+    public function dataFailToAttach()
+    {
+        return [
+            'Invalid: non-existent file' => [
+                'path' => 'thisfiledoesntexist',
+                'cid'  => 'xyz',
+            ],
+        ];
+    }
+
     /**
      * Expect exceptions on bad encoding
      */