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/core_spec.lua

@@ -220,11 +220,11 @@ describe('cmp.core', function()
                 character = 6,
               },
             },
-            newText = 'foobarbaz',
+            newText = 'AIUEO',
           },
         })
-        assert.are.same(state.buffer, { '***foobarbaz***' })
-        assert.are.same(state.cursor[2], 12)
+        assert.are.same(state.buffer, { '***AIUEO***' })
+        assert.are.same(state.cursor[2], 6)
       end)
     end)
   end)