test: refactor #6794 test

tarantool/luatest#214 adds copy directory semantics to
luatest_helpers.Server: reuse it in the scope of this test.

Follow-up #tarantool/luatest#214

NO_DOC=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit c32273ce)

test/box-luatest/gh_6794_recover_nonmatching_xlogs_test.lua

@@ -4,9 +4,8 @@ local g = t.group()
 local fio = require('fio')
 
 g.before_each(function()
-    g.server = server:new({alias = 'master'})
-    -- Initialize server's workdir with specially crafted snap/xlog pair.
-    fio.copytree('./test/box-luatest/gh_6794_data', g.server.workdir)
+    g.server = server:new({alias = 'master',
+                          datadir = 'test/box-luatest/gh_6794_data'})
 end)
 
 g.after_test("test_panic_without_force_recovery", function()