REGTESTS: add a test for the threaded Lua code

REGTESTS: add a test for the threaded Lua code

This is simply txn_get_priv.vtc with the loading made using
lua-load-per-thread, allowing all threads to run independently.
There's nothing global nor thread-specific in this test, which makes
an excellent example of something that should work out of the box.
Four concurrent clients are initialized with 4 loops each so as to
give a little chance to various threads to run concurrently.