tests: Fixes a few recently spotted test-flakes

[ptabor]

Unexpected goroutines running after all test(s).
1 instances of:
syscall.Syscall(...)
	/usr/local/go/src/syscall/asm_linux_386.s:19 +0x5
syscall.Close(...)
	/usr/local/go/src/syscall/zsyscall_linux_386.go:285 +0x3d
internal/poll.(*FD).destroy(...)
	/usr/local/go/src/internal/poll/fd_unix.go:77 +0x30
internal/poll.(*FD).decref(...)
	/usr/local/go/src/internal/poll/fd_mutex.go:213 +0x38
internal/poll.(*FD).Close(...)
	/usr/local/go/src/internal/poll/fd_unix.go:99 +0x43
net.(*netFD).Close(...)
	/usr/local/go/src/net/fd_posix.go:37 +0x49
FAIL	go.etcd.io/etcd/tests/v3/integration/client	0.039s

--- FAIL: TestServer_TCP_Secure_DelayTx (0.20s)
    server_test.go:110: took 128.026085ms with no latency
    server_test.go:125: took 62.980988ms with latency 50ms+-5ms
    server_test.go:133: expected took1 128.026085ms < took2 62.980988ms (with latency)

#12372