Enable clear/purge for failed digests

golden/go/digesttools/digesttools_test.go

 package digesttools
 
 import (
         "math"
         "testing"
 
         "github.com/stretchr/testify/assert"
         "go.skia.org/infra/golden/go/diff"
         "go.skia.org/infra/golden/go/expstorage"
         "go.skia.org/infra/golden/go/tally"
         "go.skia.org/infra/golden/go/types"
 )
 
 type MockDiffStore struct{}
 
 func (m MockDiffStore) AbsPath(digest []string) map[string]string                { return nil }
 func (m MockDiffStore) UnavailableDigests() map[string]*diff.DigestFailure       { return nil }
-func (m MockDiffStore) PurgeDigests(digests []string, purgeGS bool)              {}
+func (m MockDiffStore) PurgeDigests(digests []string, purgeGS bool) error        { return nil }
 func (m MockDiffStore) SetDigestSets(namedDigestSets map[string]map[string]bool) {}
 
 // Get always finds that digest "eee" is closest to dMain.
 func (m MockDiffStore) Get(dMain string, dRest []string) (map[string]*diff.DiffMetrics, error) {
         result := map[string]*diff.DiffMetrics{}
         for i, d := range dRest {
                 diffPercent := float32(i + 2)
                 if d == "eee" {
                         diffPercent = 0.1
                 }
@@ skipping 44 matching lines @@
         assert.InDelta(t, 0.166, float64(c.Diff), 0.01)
         assert.Equal(t, "bbb", c.Digest)
         assert.Equal(t, []int{5, 3, 4, 0}, c.MaxRGBA)
 }
 
 func TestCombinedDiffMetric(t *testing.T) {
         assert.InDelta(t, 1.0, combinedDiffMetric(0.0, []int{}), 0.000001)
         assert.InDelta(t, 1.0, combinedDiffMetric(1.0, []int{255, 255, 255, 255}), 0.000001)
         assert.InDelta(t, math.Sqrt(0.5), combinedDiffMetric(0.5, []int{255, 255, 255, 255}), 0.000001)
 }