Enable scaffold upgrade test for Composer 2

Problem/Motivation

In #3126566: Allow Drupal to work with Composer 2, the scaffold upgrade test was disabled for Composer 2. This test will fail with Composer 2 until there is a stable (tagged) release of drupal/recommended-project et. al. that can be installed with Composer 2, as the upgrade test begins by installing a fresh stable version of the Drupal project template, and then tries to upgrade it to the SUT (the Drupal system-under-test, the current Drupal version with the patch being tested applied).

Proposed resolution

Re-enable this test once there is a stable release available to start from. The test should continue to install Drupal ^8.8.0 when run with Composer 1, but should start with the later release when run with Composer 2.

The test in question is in core/tests/Drupal/Tests/Composer/Plugin/Scaffold/Functional/ScaffoldUpgradeTest.php:

   public function testScaffoldUpgrade() {
+    $composerVersionLine = exec('composer --version');
+    if (strstr($composerVersionLine, 'Composer version 2') !== FALSE) {
+      $this->markTestSkipped('We cannot run the scaffold upgrade test with Composer 2 until we have a stable version of drupal/core-composer-scaffold to start from that we can install with Composer 2.x.');
+    }

Remaining tasks

• Remove the 'markTestSkipped' in testScaffoldUpgrade.
• Select the starting version of Drupal based on the version of Composer being used to run the test.

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

n/a