[uptime] rewite to testable and add test

[wafuwafu13]

Before rewrite check-uptime.go, can't test correctly because of reference.
Here is an example.

check-uptime-test.go

func TestCheckUptime(t *testing.T) {
	tests := []struct {
		args []string
		want checkers.Status
	}{
		{
			[]string{},
			checkers.OK,
		},
		{
			[]string{"-w", "-1"},
			checkers.OK,
		},
		{
			[]string{"-W", "-1"},
			checkers.WARNING,
		},
		{
			[]string{"-c", "-1"},
			checkers.OK,
		},
		{
			[]string{"-C", "-1"},
			checkers.CRITICAL,
		},
	}
	for _, tt := range tests {
		ckr := run(tt.args)
		assert.Equal(t, tt.want, ckr.Status)
	}
}

debug fmt.print(opts) in run function

~/Desktop/go-check-plugins/check-uptime/lib
$ go test -v
=== RUN   TestCheckUptime
{<nil> <nil> <nil> <nil> <nil> <nil>}
{<nil> 0xc0000a6aa8 <nil> <nil> <nil> <nil>}
{<nil> 0xc0000a6aa8 <nil> <nil> 0xc0000a6b30 <nil>}
{<nil> 0xc0000a6aa8 0xc0000a6bb8 <nil> 0xc0000a6b30 <nil>}
    check-uptime_test.go:38: 
                Error Trace:    /Users/wafuwafu13/Desktop/go-check-plugins/check-uptime/lib/check-uptime_test.go:38
                Error:          Not equal: 
                                expected: 0
                                actual  : 1
                Test:           TestCheckUptime
{<nil> 0xc0000a6aa8 0xc0000a6bb8 <nil> 0xc0000a6b30 0xc0000a6cb8}
--- FAIL: TestCheckUptime (0.00s)
FAIL
exit status 1
FAIL    github.com/mackerelio/go-check-plugins/check-uptime/lib 0.340s

Ref: https://github.com/mackerelio/go-check-plugins/blob/master/check-tcp/lib/check-tcp.go