patch 8.1.0044: if a test function exists Vim this may go unnoticed

Problem:    If a test function exists Vim this may go unnoticed.
Solution:   Check for a test funtion quitting Vim.  Fix tests that did exit
            Vim.
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 6114728..28db289 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -198,3 +198,9 @@
   smile
   sleep 300m
 endfunc
+
+" Must be last.
+func Test_zz_quit_detected()
+  " Verify that if a test function ends Vim the test script detects this.
+  quit
+endfunc