Set $content_width in actual test function, not dataProvider

Tests failed in Travis,
so attempt to fix them.

tests/php/modules/shortcodes/test-class.dailymotion.php

@@ -154,9 +154,6 @@ public function test_shortcodes_dailymotion_params() {
 	 * @return array The test data.
 	 */
 	public function get_dailymotion_amp_data() {
-		global $content_width;
-		$content_width = self::CONTENT_WIDTH;
-
 		$id             = 26423151;
 		$default_height = 471;
 
@@ -202,6 +199,9 @@ public function get_dailymotion_amp_data() {
 	 * @param string $expected The expected return value of the function.
 	 */
 	public function test_shortcodes_dailymotion_amp( $shortcode_content, $expected ) {
+		global $content_width;
+		$content_width = self::CONTENT_WIDTH;
+
 		add_filter( 'jetpack_is_amp_request', '__return_true' );
 		$this->assertEquals( $expected, do_shortcode( $shortcode_content ) );
 	}