[chiptest] Network-independent RPC server shutdown (#15954)
[chiptest] Network-independent RPC server shutdown (#15954)

Terminating app-register RPC server with the usage RPC call itself
might cause troubles in case when the network connection is not
reliable (e.g. no route to the server). This commit simplifies the
shutdown process by using dedicated shutdown method.
project-chip/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip