digestset: refine some words on unit test

1. when lookup an entry which is missing, it should say NotFound.
2. when add duplicated entry, the entries size should be increased.
3. when add entry which has different algorithm, it should be allowed.

Signed-off-by: zhouhaibing089 <zhouhaibing089@gmail.com>

digestset/set_test.go

@@ -51,7 +51,7 @@ func TestLookup(t *testing.T) {
 
 	dgst, err = dset.Lookup("9876")
 	if err == nil {
-		t.Fatal("Expected ambiguous error looking up: 9876")
+		t.Fatal("Expected not found error looking up: 9876")
 	}
 	if err != ErrDigestNotFound {
 		t.Fatal(err)
@@ -118,15 +118,15 @@ func TestAddDuplication(t *testing.T) {
 	}
 
 	if len(dset.entries) != 8 {
-		t.Fatal("Duplicate digest insert allowed")
+		t.Fatal("Duplicate digest insert should not increase entries size")
 	}
 
 	if err := dset.Add(digest.Digest("sha384:123451111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111")); err != nil {
 		t.Fatal(err)
 	}
 
 	if len(dset.entries) != 9 {
-		t.Fatal("Insert with different algorithm not allowed")
+		t.Fatal("Insert with different algorithm should be allowed")
 	}
 }