Currently many Tkinter tests depends on tkinter._default_root. I.e. they reuse the same Tcl interpreter and main window. This can cause unexpected dependencies between tests. Proposed patch creates new root for every test, this makes tests mutually independent. It also fixes some bugs in NoDefaultRoot mode and get rid of 'can't invoke "event" command:' messages in tests. This patch is needed to run Tkinter tests in different "wantobjects" modes (issue21585).