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.

src/testdir/test_assert.vim

@@ -198,3 +198,9 @@ func Test_user_is_happy()
   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