rpctest: use errgroup to catch goroutine errors

[dajohi]

No description provided.

[dajohi]

rpctest/rpc_harness_test.go:371:2: SA2002: the goroutine calls T.Fatalf, which must be called in the same goroutine as the test (staticcheck)
	go func() {
	^
rpctest/rpc_harness_test.go:393:2: SA2002: the goroutine calls T.Fatalf, which must be called in the same goroutine as the test (staticcheck)
	go func() {
	^
rpctest/rpc_harness_test.go:446:2: SA2002: the goroutine calls T.Fatalf, which must be called in the same goroutine as the test (staticcheck)
	go func() {
	^