time: make median time library type private (#6853)

This is a very minor change, but I was looking through the code, and
this seems like it shouldn't be exported or used more broadly, so I've
moved it out.

state/time_test.go

@@ -0,0 +1,57 @@
+package state
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	tmtime "github.com/tendermint/tendermint/libs/time"
+)
+
+func TestWeightedMedian(t *testing.T) {
+	m := make([]*weightedTime, 3)
+
+	t1 := tmtime.Now()
+	t2 := t1.Add(5 * time.Second)
+	t3 := t1.Add(10 * time.Second)
+
+	m[2] = newWeightedTime(t1, 33) // faulty processes
+	m[0] = newWeightedTime(t2, 40) // correct processes
+	m[1] = newWeightedTime(t3, 27) // correct processes
+	totalVotingPower := int64(100)
+
+	median := weightedMedian(m, totalVotingPower)
+	assert.Equal(t, t2, median)
+	// median always returns value between values of correct processes
+	assert.Equal(t, true, (median.After(t1) || median.Equal(t1)) &&
+		(median.Before(t3) || median.Equal(t3)))
+
+	m[1] = newWeightedTime(t1, 40) // correct processes
+	m[2] = newWeightedTime(t2, 27) // correct processes
+	m[0] = newWeightedTime(t3, 33) // faulty processes
+	totalVotingPower = int64(100)
+
+	median = weightedMedian(m, totalVotingPower)
+	assert.Equal(t, t2, median)
+	// median always returns value between values of correct processes
+	assert.Equal(t, true, (median.After(t1) || median.Equal(t1)) &&
+		(median.Before(t2) || median.Equal(t2)))
+
+	m = make([]*weightedTime, 8)
+	t4 := t1.Add(15 * time.Second)
+	t5 := t1.Add(60 * time.Second)
+
+	m[3] = newWeightedTime(t1, 10) // correct processes
+	m[1] = newWeightedTime(t2, 10) // correct processes
+	m[5] = newWeightedTime(t2, 10) // correct processes
+	m[4] = newWeightedTime(t3, 23) // faulty processes
+	m[0] = newWeightedTime(t4, 20) // correct processes
+	m[7] = newWeightedTime(t5, 10) // faulty processes
+	totalVotingPower = int64(83)
+
+	median = weightedMedian(m, totalVotingPower)
+	assert.Equal(t, t3, median)
+	// median always returns value between values of correct processes
+	assert.Equal(t, true, (median.After(t1) || median.Equal(t1)) &&
+		(median.Before(t4) || median.Equal(t4)))
+}