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/screenchar_spec.lua

@@ -32,7 +32,7 @@ end
 
 describe('screenchar() and family respect floating windows', function()
   local function with_ext_multigrid(multigrid)
-    before_each(function()
+    setup(function()
       clear()
       Screen.new(40, 7, { ext_multigrid = multigrid })
       -- These commands result into visible text `aabc`.