[SUREFIRE-2098] ParallelComputerUtilTest flaky test

**[Olivier Lamy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=olamy)** opened **[SUREFIRE-2098](https://issues.apache.org/jira/browse/SUREFIRE-2098?redirect=false)** and commented

the test ParallelComputerUtilTest made assumptions which can depend on node.

```java
Error: org.apache.maven.surefire.junitcore.pc.ParallelComputerUtilTest.withoutShutdown Time elapsed: 11.846 s <<< ERROR!
2477java.lang.AssertionError: expected:<10000.0> but was:<10502.0>
2478 at org.junit.Assert.fail(Assert.java:91)
2479 at org.junit.Assert.failNotEquals(Assert.java:645)
2480 at org.junit.Assert.assertEquals(Assert.java:441)
2481 at org.junit.Assert.assertEquals(Assert.java:510)
2482 at org.apache.maven.surefire.junitcore.pc.ParallelComputerUtilTest.withoutShutdown(ParallelComputerUtilTest.java:956)
```

or delta should be increase

---
No further details from [SUREFIRE-2098](https://issues.apache.org/jira/browse/SUREFIRE-2098?redirect=false)

Start with org.apache.maven.surefire.junitcore.pc.ParallelComputerUtilTest, especially withoutShutdown at line 956, and reproduce the reported assertion failure. Inspect how the test derives its expected and actual values, then make the test independent of node-dependent timing; done means the test passes reliably across nodes.

Written by the indexing model from the issue text.