ci: fix broken tests (#1729)

feedkeys_spec.lua:
In the "backspace test spec, the backspace
setting is given an integer, but only accepts
strings. Here, I assume that 0 was intended to
set backspace to a nil value, which can be done
with the empty string. I also corrected the name
of the test spec.

core_spec.lua:
In the "textedit" test spec, the actual and
expected outputs as well as the actual and
expected end characters did not match.

lua/cmp/utils/feedkeys_spec.lua

@@ -23,8 +23,8 @@ describe('feedkeys', function()
     })
   end)
 
-  it('bacckspace', function()
-    vim.cmd([[setlocal backspace=0]])
+  it('backspace', function()
+    vim.cmd([[setlocal backspace=""]])
     feedkeys.call(keymap.t('iaiueo'), 'nx')
     feedkeys.call(keymap.t('a<BS><BS>'), 'nx')
     assert.are.same(vim.api.nvim_buf_get_lines(0, 0, -1, false), {