test: move txn_proxy.lua to box/lua

txn_proxy is a special utility for transaction tests.
Formerly it was used only for vinyl tests and thus was placed in
vinyl folder.
Now the time has come to test memtx transactions and the utility
must be placed amongst other utils - in box/lua.

Needed for #4897

test/vinyl/suite.ini

@@ -4,7 +4,7 @@ description = vinyl integration tests
 script = vinyl.lua
 release_disabled = errinj.test.lua errinj_ddl.test.lua errinj_gc.test.lua errinj_stat.test.lua errinj_tx.test.lua errinj_vylog.test.lua partial_dump.test.lua quota_timeout.test.lua recovery_quota.test.lua replica_rejoin.test.lua gh-4864-stmt-alloc-fail-compact.test.lua gh-4805-open-run-err-recovery.test.lua gh-4821-ddl-during-throttled-dump.test.lua gh-3395-read-prepared-uncommitted.test.lua
 config = suite.cfg
-lua_libs = suite.lua stress.lua large.lua txn_proxy.lua ../box/lua/utils.lua
+lua_libs = suite.lua stress.lua large.lua ../box/lua/txn_proxy.lua ../box/lua/utils.lua
 use_unix_sockets = True
 use_unix_sockets_iproto = True
 long_run = stress.test.lua large.test.lua write_iterator_rand.test.lua dump_stress.test.lua select_consistency.test.lua throttle.test.lua