Discard working copy of CU as often as possible #1344

The method `discardWorkingCopy()` must be called as often as the
`becomeWorkingcopy()`.

Author: ptziegler

org.eclipse.wb.tests/src/org/eclipse/wb/tests/designer/core/AbstractJavaProjectTest.java

@@ -267,7 +267,9 @@ public final ICompilationUnit createModelCompilationUnit(String packageName,
 		IPackageFragment pkg = m_testProject.getPackage(packageName);
 		// discard cached unit
 		ICompilationUnit compilationUnit = pkg.getCompilationUnit(unitName);
-		compilationUnit.discardWorkingCopy();
+		while (compilationUnit.isWorkingCopy()) {
+			compilationUnit.discardWorkingCopy();
+		}
 		// create unit
 		compilationUnit = m_testProject.createUnit(pkg, unitName, code);
 		IFile resource = (IFile) compilationUnit.getUnderlyingResource();