[3.6] bpo-30303: IDLE test_textview: add comments and test, increase coverage to…

[Mariatta]

… 100% (GH-1641).

(cherry picked from commit 295304d)

[terryjreedy]

For bpo-30303, PR 1499 was applied first and then PR 1641 on top of it. This backport of 1641 to 3.6 was prepared without a backport of 1499 having been applied to 3.6. The result was not only the overt merge conflict noted above (after I merged PR 1966, the backport of 1499) but at least one non-conflict error that I noticed. 1499 added a new class:
class ButtonClickTextViewTest(unittest.TestCase):
1641 merely added a comment and changed the name:
-class ButtonClickTextViewTest(unittest.TestCase):
+# Call TextViewer with _utest=True.
+class ButtonClickTest(unittest.TestCase):+# Call TextViewer with _utest=True.
But this PR, prepared without the new class present in the file, has a quit different diff. It adds the new class as ButtonClickTest. When merged into 3.6 with ButtonClickTextViewTest present the result is two copies of the class with different names, which is wrong.

I considered trying to fix both files by copying the 3.7 versions into 3.7, but the result commit would have included 20 other files, and at best the result would have been two 'wrong' commits to get one 'right' diff. So I think it best to redo the backport with 1499/1966 already applied.

[terryjreedy]

The replacement backport is
#2018