Replace assertions involving calls to file_exists with assertFileExists()/assertFileNotExists() or assertDirectoryExists()/assertDirectoryNotExists()

file_exists() also returns TRUE for directories, so the tests can be more accurate here about what they are looking for.

+++ b/core/modules/simpletest/src/TestBase.php
@@ -734,7 +734,7 @@ protected function assertIdenticalObject($object1, $object2, $message = '', $gro
-    return $this->assertFalse(file_exists(DRUPAL_ROOT . '/' . $this->siteDirectory . '/error.log'), 'PHP error.log is empty.');
+    return $this->assertFileNotExists(DRUPAL_ROOT . '/' . $this->siteDirectory . '/error.log');

I don't think this exists in SimpleTest - the patch is still green because this won't be called any more by any core tests, but contrib might still use it.

1. +++ b/core/tests/Drupal/FunctionalTests/Installer/InstallerCustomConfigDirectoryCreateTest.php
   @@ -38,12 +38,11 @@ protected function prepareEnvironment() {
   -    $this->assertTrue(file_exists($this->publicFilesDirectory . '/config_custom') && is_dir($this->publicFilesDirectory . '/config_custom'), "The directory {$this->publicFilesDirectory}/custom_config exists.");
   +    $this->assertFileExists($this->publicFilesDirectory . '/config_custom') && is_dir($this->publicFilesDirectory . '/config_custom');
   

   This isn't quite right, this needs to become assertDirectoryExists()

2. +++ b/core/tests/Drupal/FunctionalTests/Installer/InstallerCustomConfigDirectoryCreateTest.php
   @@ -38,12 +38,11 @@ protected function prepareEnvironment() {
   -    $this->assertTrue(file_exists($sync_directory) && is_dir($sync_directory), "The directory {$sync_directory} exists.");
   -
   +    $this->assertFileExists($sync_directory) && is_dir($sync_directory);
   

   Same here.

No worries, thanks for working on this - all looks good now.

The 8.9.x patch here has some bad code, opened #3133103: [8.9 only] InstallerCustomConfigDirectoryCreateTest has bad assertions to address this.