patch 8.0.0440: not enough test coverage in Insert mode

Problem:    Not enough test coverage in Insert mode.
Solution:   Add lots of tests.  Add test_override(). (Christian Brabandt,
            closes #1521)
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index d750da5..732cc0e 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -49,7 +49,7 @@
 " This also enables use of line continuation.
 set nocp viminfo+=nviminfo
 
-" Use utf-8 or latin1 be default, instead of whatever the system default
+" Use utf-8 or latin1 by default, instead of whatever the system default
 " happens to be.  Individual tests can overrule this at the top of the file.
 if has('multi_byte')
   set encoding=utf-8
@@ -96,6 +96,9 @@
   " mode message.
   set noshowmode
 
+  " Clear any overrides.
+  call test_override('ALL', 0)
+
   if exists("*SetUp")
     try
       call SetUp()