testing: fix goroutine leak in TestClientUpdatesParamsAfterGoAway

[charlesxsh]

If test failed at log like following:

    clientconn_test.go:xxx: Dial(127.0.0.1:34491, _) = _, context deadline exceeded, want _, <nil>

goroutine will be blocked at

go func() {
...
     <-connected
...
}

goroutine 8 [chan receive]:
google.golang.org/grpc.TestClientUpdatesParamsAfterGoAway.func1(0xab6d80, 0xc00000c8b8, 0xc0002c0a00, 0xc000028900)
	/fuzz/target/clientconn_test.go:978 +0x30f
created by google.golang.org/grpc.TestClientUpdatesParamsAfterGoAway
	/fuzz/target/clientconn_test.go:956 +0x212

--- FAIL: TestClientUpdatesParamsAfterGoAway (5.00s)
FAIL

RELEASE NOTES: none

[dfawley]

This would be a little neater (taking advantage of a grpc internal feature):

connected := grpcsync.NewEvent()
defer connected.Fire()
...
<-connected.Done()
...
connected.Fire()

[charlesxsh]

nice fix! updated @dfawley

[github-actions]

This PR is labeled as requiring an update from the reporter, and no update has been received after 6 days. If no update is provided in the next 7 days, this issue will be automatically closed.

[dfawley]

Thank you for the fix!