Try using DIRECTORY_SEPARATOR

Author: swissspidy

src/Context/FeatureContext.php

@@ -1042,7 +1042,7 @@ private static function get_event_file( $scope, &$line ): ?string {
 	 */
 	public function create_run_dir(): void {
 		if ( ! isset( $this->variables['RUN_DIR'] ) ) {
-			self::$run_dir              = sys_get_temp_dir() . '/' . uniqid( 'wp-cli-test-run-' . self::$temp_dir_infix . '-', true );
+			self::$run_dir              = sys_get_temp_dir() . DIRECTORY_SEPARATOR . uniqid( 'wp-cli-test-run-' . self::$temp_dir_infix . '-', true );
 			$this->variables['RUN_DIR'] = self::$run_dir;
 			mkdir( $this->variables['RUN_DIR'] );
 		}

src/Context/ThenStepDefinitions.php

@@ -456,8 +456,8 @@ public function then_a_specific_file_folder_should_exist( $path, $type, $strictl
 		$path = $this->replace_variables( $path );
 
 		// If it's a relative path, make it relative to the current test dir.
-		if ( '/' !== $path[0] ) {
-			$path = $this->variables['RUN_DIR'] . "/$path";
+		if ( DIRECTORY_SEPARATOR !== $path[0] ) {
+			$path = $this->variables['RUN_DIR'] . DIRECTORY_SEPARATOR . $path;
 		}
 
 		$exists = static function ( $path ) use ( $type ) {