Update tests with output

[GromNaN]

Some tests generate output.
This is spotted by PHPUnit by setting beStrictAboutOutputDuringTests="true".

We need alternative ways to get the output (PHPUnit extension? TestDox?) and add assertions on this values.

1) Doctrine\ODM\MongoDB\Tests\Functional\MemoryUsageTest::testMemoryUsage
Test code or tested code printed unexpected output: Memory increased by 8.05 kb

/home/runner/work/mongodb-odm/mongodb-odm/tests/Tests/Functional/MemoryUsageTest.php:29

2) Doctrine\Tests\ODM\MongoDB\Performance\HydrationPerformanceTest::testHydrationPerformance
Test code or tested code printed unexpected output: Memory usage before: 59018.359375 KB
Memory usage after: 59531.4765625 KB
Hydrated 10000 objects in 0.42882490158081 seconds

/home/runner/work/mongodb-odm/mongodb-odm/tests/Tests/Performance/HydrationPerformanceTest.php:25

3) Doctrine\Tests\ODM\MongoDB\Performance\InsertPerformanceTest::testInsertPerformance
Test code or tested code printed unexpected output: Memory usage before: 59585.703125 KB
Memory usage after: 59560.2265625 KB
Inserted 10000 objects in 0.3379430770874 seconds

/home/runner/work/mongodb-odm/mongodb-odm/tests/Tests/Performance/InsertPerformanceTest.php:25

4) Doctrine\ODM\MongoDB\Tests\Performance\MemoryUsageTest::testMemoryUsage
Test code or tested code printed unexpected output: Memory increased by 8.05 kb

/home/runner/work/mongodb-odm/mongodb-odm/tests/Tests/Performance/MemoryUsageTest.php:31

5) Doctrine\Tests\ODM\MongoDB\Performance\UnitOfWorkPerformanceTest::testComputeChanges
Test code or tested code printed unexpected output: Compute ChangeSet 10000 objects in 2.4595119953156 seconds

/home/runner/work/mongodb-odm/mongodb-odm/tests/Tests/Performance/UnitOfWorkPerformanceTest.php:24

[alcaeus]

These tests are old. We could either add assertions to act as a canary when memory usage goes up, or we could try to rely on the benchmarks for this.