Only simulate sleeping in "TestBackoff_Exponential".

Add another test that (shortly) sleeps.

backoff.go

@@ -37,6 +37,8 @@ type exponentialBackoff struct {
 	initial  time.Duration
 	maxWait  time.Duration
 	nextWait time.Duration
+
+	getContextWithTimeout func(parent context.Context, timeout time.Duration) (context.Context, context.CancelFunc)
 }
 
 func NewExponentialBackoff(initial time.Duration, maxWait time.Duration) (Backoff, error) {
@@ -52,6 +54,8 @@ func NewExponentialBackoff(initial time.Duration, maxWait time.Duration) (Backof
 		maxWait: maxWait,
 
 		nextWait: initial,
+
+		getContextWithTimeout: context.WithTimeout,
 	}, nil
 }
 
@@ -64,7 +68,7 @@ func (b *exponentialBackoff) NextWait() time.Duration {
 }
 
 func (b *exponentialBackoff) Wait(ctx context.Context) {
-	waiter, cancel := context.WithTimeout(ctx, b.nextWait)
+	waiter, cancel := b.getContextWithTimeout(ctx, b.nextWait)
 	defer cancel()
 
 	b.nextWait = b.nextWait * 2

backoff_test.go

@@ -31,7 +31,6 @@ import (
 )
 
 func TestBackoff_Exponential(t *testing.T) {
-	t.Parallel()
 	assert := assert.New(t)
 	minWait := 100 * time.Millisecond
 	backoff, err := NewExponentialBackoff(minWait, 500*time.Millisecond)
@@ -47,14 +46,27 @@ func TestBackoff_Exponential(t *testing.T) {
 
 	for _, wait := range waitTimes {
 		assert.Equal(wait, backoff.NextWait())
-
-		a := time.Now()
+		backoff.(*exponentialBackoff).getContextWithTimeout = func(parent context.Context, timeout time.Duration) (context.Context, context.CancelFunc) {
+			assert.Equal(wait, timeout)
+			return context.WithTimeout(parent, time.Millisecond)
+		}
 		backoff.Wait(context.Background())
-		b := time.Now()
-		assert.GreaterOrEqual(b.Sub(a), wait)
 	}
 
 	backoff.Reset()
+	backoff.(*exponentialBackoff).getContextWithTimeout = func(parent context.Context, timeout time.Duration) (context.Context, context.CancelFunc) {
+		assert.Equal(minWait, timeout)
+		return context.WithTimeout(parent, time.Millisecond)
+	}
+	backoff.Wait(context.Background())
+}
+
+func TestBackoff_ExponentialRealSleep(t *testing.T) {
+	assert := assert.New(t)
+	minWait := 100 * time.Millisecond
+	backoff, err := NewExponentialBackoff(minWait, 500*time.Millisecond)
+	require.NoError(t, err)
+
 	a := time.Now()
 	backoff.Wait(context.Background())
 	b := time.Now()