test-privserver-client: wait for server to die between iterations

test-privserver-client: wait for server to die between iterations

This fixes a race condition: the server exits while the client continues
to the next iteration. If the server wins, the test passes. If the client
wins, it sends a message to the dying service, never gets a reply, and the
test fails.

My branch to refactor the main loop for fd.o #23194 seems to make the
client more likely to win this race, resulting in intermittent test
failures.

This is an instance of the general problem described by fd.o #11454.