patch 8.2.0400: not all tests using a terminal are in the list of flaky tests

Problem:    Not all tests using a terminal are in the list of flaky tests.
Solution:   Introduce the test_is_flaky flag.
diff --git a/src/testdir/screendump.vim b/src/testdir/screendump.vim
index d04d96d..14863f9 100644
--- a/src/testdir/screendump.vim
+++ b/src/testdir/screendump.vim
@@ -24,6 +24,9 @@
   let reference = 'dumps/' . a:filename . '.dump'
   let testfile = 'failed/' . a:filename . '.dump'
 
+  " Starting a terminal to make a screendump is always considered flaky.
+  let test_is_flaky = 1
+
   " Redraw to execute the code that updates the screen.  Otherwise we get the
   " text and attributes only from the internal buffer.
   redraw