AssertButtonsTrait has invalid PHP syntax

Problem/Motivation

Discovered via PHPStan in #3178534: Start running PHPStan on Drupal core (level 0), there is invalid PHP syntax in \Drupal\Tests\node\Functional\AssertButtonsTrait.

$ php -l ./core/modules/node/tests/src/Functional/AssertButtonsTrait.php
PHP Parse error:  syntax error, unexpected '+', expecting :: (T_PAAMAYIM_NEKUDOTAYIM) in ./core/modules/node/tests/src/Functional/AssertButtonsTrait.php on line 30

The line in question is

$this->assertSession()->elementTextEquals('xpath', "(//div[@class='dropbutton-wrapper']//input[@type='submit'])[{$i + 1}]", $buttons[$i]);

Only variables, array elements or object properties can be accessed using the {} syntax inside strings; other expressions are not allowed.

This was last changed in #3203476: Convert assertions involving use of xpath on divs to WebAssert, and although we don't use this trait in core (two uses in contrib), I don't know how this slipped through as I thought we had a PHP lint step in DrupalCI?

Steps to reproduce

Proposed resolution

Fix the syntax.
Add a basic test that uses this trait.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet