Revert "test: disable shada"

This reverts commit d0673ea.

Testing whether unsetting shada caused the following error in Appveyor

[00:11:35] [  ERROR   ] 1 error, listed below:
[00:11:35] [  ERROR   ] C:/projects/neovim/test/functional\core\job_spec.lua @ 661: pty process teardown does not prevent/delay exit. neovim#4798 neovim#4900
[00:11:35] .\test\functional\ui\screen.lua:302: Row 1 did not match.
[00:11:35] Expected:
[00:11:35]   |*                              |
[00:11:35]   |                              |
[00:11:35]   |                              |
[00:11:35]   |                              |
[00:11:35]   |^[Process exited 0]            |
[00:11:35]   |                              |
[00:11:35] Actual:
[00:11:35]   |*dir\api\buffer.c.gcda:Data fil|
[00:11:35]   |e mismatch - some data files m|
[00:11:35]   |ay have been concurrently upda|
[00:11:35]   |t                             |
[00:11:35]   |^[Process exited -1073741819]  |
[00:11:35]   |                              |
[00:11:35]
[00:11:35] To print the expect() call that would assert the current screen state, use
[00:11:35] screen:snapshot_util(). In case of non-deterministic failures, use
[00:11:35] screen:redraw_debug() to show all intermediate screen states.
[00:11:35]
[00:11:35] stack traceback:
[00:11:35] 	.\test\functional\ui\screen.lua:302: in function 'wait'
[00:11:35] 	.\test\functional\ui\screen.lua:216: in function 'expect'
[00:11:35] 	C:/projects/neovim/test/functional\core\job_spec.lua:672: in function <C:/projects/neovim/test/functional\core\job_spec.lua:661>

test/functional/core/job_spec.lua

@@ -665,7 +665,7 @@ describe("pty process teardown", function()
     if helpers.iswin() then
       command([[set shellcmdflag=/s/c shellxquote=\"]])
       feed_command(':terminal "'..helpers.nvim_prog
-        ..'" -u NONE -i NONE --cmd "'..nvim_set..' shada='
+        ..'" -u NONE -i NONE --cmd "'..nvim_set
         -- Use `ping` to emulate Unix's `sleep`
         ..'" -c terminal -c "!start /b cmd /s/c ""ping 127.0.0.1 -n 301 > nul""" -c qa')
       feed('G')