The following test cases have infrequent multiple fails in the same I-build:
* org.eclipse.jdt.text.tests.contentassist.TypeCompletionTest
* org.eclipse.jdt.text.tests.contentassist.JavadocCompletionTest
* org.eclipse.jdt.text.tests.contentassist.CodeCompletionTest
These test cases open a Java editor and so should wait on the Java reconciler to finish.
Otherwise the reconciler can run JDT model operations during subsequent tests.
This change adds such waiting in AbstractCompletionTest.
See: https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/735