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/uniq_spec.lua

@@ -7,7 +7,7 @@ local command = n.command
 local exc_exec = n.exc_exec
 local pcall_err = t.pcall_err
 
-before_each(clear)
+setup(clear)
 
 describe('uniq()', function()
   it('errors out when processing special values', function()