fix flaky test: testObserverMethodsInParentOfAlternativeAndSpecializedBeans

[ljmcr]

Running command:
mvn -pl webbeans-impl edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=org.apache.webbeans.test.specalization.observer.pub.PublicObserverTest#testObserverMethodsInParentOfAlternativeAndSpecializedBeans -DnondexRuns=10

Original error:

error stack trace:

Specific line of code: Line 172 at SpecializationUtil.java:

2. What I am trying to do:
   Adding a hashset to store all the elements that should be remove in beanAttributeMap, and remove all those elements in the
   set together after looping through the whole map. This will avoid removing elements while looping with iteratorm, which
   may cause IndexOutOfBoundException or ConcurrentModificationException when doing parallel programming .
3. I used hashset to store the elements since the hashset is better on adding/deleting elements compare to ArrayList, which will
   help reduce the time complexity.