legacy2luatest: Use before_each instead of setup.

This will avoid confusion if additional `it()` blocks are added.
(`setup()` only runs once per `describe()` block, whereas `before_each()`
runs before each `it()`).

scripts/legacy2luatest.pl

@@ -287,7 +287,7 @@ local feed, insert, source = helpers.feed, helpers.insert, helpers.source
 local clear, execute, expect = helpers.clear, helpers.execute, helpers.expect
 
 describe('$test_name', function()
-  setup(clear)
+  before_each(clear)
 
   it('is working', function()
 @{[join "\n", map { /^$/ ? '' : '    ' . $_ } @{$test_body_lines}]}