[TESTBUG] CgroupMetricsTester.testMemoryUsage() test is unreliable

In the family of cgroup tests there is test "TestCgroupMetrics.java" which fails with some probability here and then.

The failures are more frequent on older JVMs (e.g. 8) where G1 is not a default GC. But the failure can be easily reproduced with G1 as well.

*How to reproduce*
Run TestCgroupMetrics.java, but modify it a bit. In method testAll() of class MetricsTestser, run "tester.testMemoryUsage();" in a loop:
```
- tester.testMemoryUsage();
+ for (int i = 0; i < 1000; i++)
+ {
+ tester.testMemoryUsage();
+ }
```