test: set shutdown timeout to infinity for default luatest instance

With the default shutdown timeout of 3 seconds, a test that leaves
behind asynchronous requests would still pass, but it would take longer
to finish, because the server instance started by Tarantool would have
to wait for the dangling requests to complete. Setting the timeout to
infinity will result in a hang, making us fix the test.

Infinite timeout is also good for catching bugs like #7225 and #7256.

We don't set the timeout for diff and TAP tests because those are
deprecated and shouldn't be used for writing new tests. Nevertheless,
I manually checked that none of them hangs if the timeout is set to
infinity.

Closes #6820

NO_DOC=test
NO_CHANGELOG=test