DatabaseUpdateTestCase::testExpressionUpdate assertion on number of affected rows fails in PostgreSQL

Problem/Motivation

testExpressionUpdate

fail: [Other] Line 878 of modules/simpletest/tests/database_test.test:
Number of affected rows are returned.

  /**
   * Test updating with expressions.
   */
  function testExpressionUpdate() {
    // Set age = 1 for a single row for this test to work.
    db_update('test')
      ->condition('id', 1)
      ->fields(array('age' => 1))
      ->execute();

    // Ensure that expressions are handled properly.  This should set every
    // record's age to a square of itself, which will change only three of the
    // four records in the table since 1*1 = 1. That means only three records
    // are modified, so we should get back 3, not 4, from execute().
    $num_rows = db_update('test')
      ->expression('age', 'age * age')
      ->execute();
    $this->assertIdentical($num_rows, 3, 'Number of affected rows are returned.' . " num_rows: $num_rows");

Steps to reproduce

With PostgreSQL $num_rows is 4 so the test fails.

Proposed resolution

Probably backport changes made in #805858: Affected rows inconsistent across database engines