To reproduce:

• build extension
• load extension
• run "Add Tests" (once) -- this will print "added no tests", but you will get a partial tree in the test explorer
• run "Add Tests" (again) -- this will print "added one test", and you will not get your test node in the tree

This bug does not appear to happen if the first time you create the tree, you have the right number of siblings. To reproduce this, set alwaysChildren to true -- this will insert TEST0, and then every time you run "Add Tests", it will either add or remove TEST1.