test: reduce some clear() calls

Use only a single clear() call in some test/functional/vimscript/ test
files whose test cases have very little side effect.

A downside of using a single clear() is that if a crash happens in one
test case, all following test cases in the same file will also fail, but
these functionalities and tests don't change very often.

test/functional/plugin/shada_spec.lua

@@ -47,7 +47,7 @@ local wshada_tmp, _, fname_tmp = get_shada_rw('Xtest-functional-plugin-shada.sha
 
 describe('autoload/shada.vim', function()
   local epoch = os.date('%Y-%m-%dT%H:%M:%S', 0)
-  before_each(function()
+  setup(function()
     reset()
     nvim_command([[
     function ModifyVal(val)