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/vimscript/operators_spec.lua

@@ -6,7 +6,7 @@ local eval = n.eval
 local clear = n.clear
 
 describe('Division operator', function()
-  before_each(clear)
+  setup(clear)
 
   it('returns infinity on {positive}/0.0', function()
     eq("str2float('inf')", eval('string(1.0/0.0)'))