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/test_search.vim b/src/testdir/test_search.vim
index 7f4f7e0..730e683 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -7,7 +7,7 @@
   endif
   " need to disable char_avail,
   " so that expansion of commandline works
-  call test_disable_char_avail(1)
+  call test_override("char_avail", 1)
   new
   call setline(1, ['  1', '  2 these', '  3 the', '  4 their', '  5 there', '  6 their', '  7 the', '  8 them', '  9 these', ' 10 foobar'])
   " Test 1
@@ -194,7 +194,7 @@
   call assert_equal('  3 the', getline('.'))
 
   " clean up
-  call test_disable_char_avail(0)
+  call test_override("char_avail", 0)
   bw!
 endfunc
 
@@ -204,7 +204,7 @@
   endif
   " need to disable char_avail,
   " so that expansion of commandline works
-  call test_disable_char_avail(1)
+  call test_override("char_avail", 1)
   new
   call setline(1, ['  1', '  2 these', '  3 the theother'])
   " Test 1
@@ -266,7 +266,7 @@
 
   " clean up
   set noincsearch
-  call test_disable_char_avail(0)
+  call test_override("char_avail", 0)
   bw!
 endfunc