Fix the issue that the ChildRules will corrupt the RuleSets of a validator passed to the SetValidator

[sumtec]

Fixes #1981

[dnfadmin]

All CLA requirements met.

[JeremySkinner]

Hi @sumtec thanks for this. It looks like the new test is currently failing - could you take a look at that please? It looks like everything in ruleset b is still being executed, even though only a is selected for inclusion.

  Failed FluentValidation.Tests.ChildRulesTests.ChildRules_works_with_SetValidator_and_RuleSet [3 ms]
  Error Message:
   Assert.Equal() Failure
Expected: 1
Actual:   2
  Stack Trace:
     at FluentValidation.Tests.TestExtensions.ShouldEqual(Object actual, Object expected) in /home/runner/work/FluentValidation/FluentValidation/src/FluentValidation.Tests/TestExtensions.cs:line 27
   at FluentValidation.Tests.ChildRulesTests.ChildRules_works_with_SetValidator_and_RuleSet() in /home/runner/work/FluentValidation/FluentValidation/src/FluentValidation.Tests/ChildRulesTests.cs:line 83

[sumtec]

Hi @sumtec thanks for this. It looks like the new test is currently failing - could you take a look at that please? It looks like everything in ruleset b is still being executed, even though only a is selected for inclusion.

  Failed FluentValidation.Tests.ChildRulesTests.ChildRules_works_with_SetValidator_and_RuleSet [3 ms]
  Error Message:
   Assert.Equal() Failure
Expected: 1
Actual:   2
  Stack Trace:
     at FluentValidation.Tests.TestExtensions.ShouldEqual(Object actual, Object expected) in /home/runner/work/FluentValidation/FluentValidation/src/FluentValidation.Tests/TestExtensions.cs:line 27
   at FluentValidation.Tests.ChildRulesTests.ChildRules_works_with_SetValidator_and_RuleSet() in /home/runner/work/FluentValidation/FluentValidation/src/FluentValidation.Tests/ChildRulesTests.cs:line 83

Sorry that the last check-in seems to have some issues. Neither the test case nor the source code is the correct one.
I've made a new commit to fix the files.

[JeremySkinner]

Thanks! I'll aim to get a release out over the weekend.