[core] Fix flaky LatticeRelationTest

[219sansim]

Describe the PR

Problem
The LatticeRelationTest.testToString is detected as flaky. The following lines of code iterate over vertices which is of Type Set.

pmd/pmd-core/src/main/java/net/sourceforge/pmd/util/GraphUtil.java

Line 46 in f030d7f

for (V node : vertices) {

pmd/pmd-core/src/main/java/net/sourceforge/pmd/util/GraphUtil.java

Line 59 in f030d7f

for (V node : vertices) {

Java does not iterate over elements of a set in a particular order. This causes the output toString function to be non-deterministic and results in the LatticeRelationTest.testToString to be flaky.
Solution
I have modified the code such that we iterate over a sorted list of Vertex objects. This results in the toString output being deterministic. I had to modify the expected string in the test assertion to account for the sorting.
Another possible fix would be to modify the test itself and change the assert to expect multiple different string ordering values.

Steps to reproduce the flaky test

Setup the NonDex tool and run

mvn -pl pmd-core edu.illinois:nondex-maven-plugin:2.1.1:nondex -Dtest=net.sourceforge.pmd.lang.rule.internal.LatticeRelationTest#testToString

Error Message

Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.077 s <<< FAILURE! - in net.sourceforge.pmd.lang.rule.internal.LatticeRelationTest
[ERROR] net.sourceforge.pmd.lang.rule.internal.LatticeRelationTest.testToString  Time elapsed: 0.047 s  <<< FAILURE!
org.opentest4j.AssertionFailedError: 
expected: <strict digraph {
n0 [ shape=box, color=green, label="[]" ];
n1 [ shape=box, color=green, label="[1]" ];
n2 [ shape=box, color=green, label="[2]" ];
n3 [ shape=box, color=black, label="[1, 2]" ];
n1 -> n0;
n2 -> n0;
n3 -> n0;
n3 -> n1;
n3 -> n2;
}> but was: <strict digraph {
n0 [ shape=box, color=green, label="[2]" ];
n1 [ shape=box, color=green, label="[]" ];
n2 [ shape=box, color=black, label="[1, 2]" ];
n3 [ shape=box, color=green, label="[1]" ];
n0 -> n1;
n2 -> n0;
n2 -> n1;
n2 -> n3;
n3 -> n1;
}>

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)

[pmd-test]

	1 Message
📖	Compared to master: This changeset changes 0 violations, introduces 0 new violations, 0 new errors and 0 new configuration errors, removes 0 violations, 0 errors and 0 configuration errors. Download full report as build artifact
✅	Compared to master: This changeset changes 0 violations, introduces 0 new violations, 0 new errors and 0 new configuration errors, removes 0 violations, 0 errors and 0 configuration errors. Download full report as build artifact

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[oowekyala]

Thank you for identifying and fixing this!