fix: compare legacy event timestamps by value instead of format (#50)

Legacy event timestamp validation was done by string comparison, but
this requires each language to format the timestamp in the same way.
In Go, sub-second precision isn't always included [1]; in the test
events the seconds field is 0 but is written with sub-second precision.

It's not feasible to change the marshalling of timestamps in the Go test
function, so instead we'll compare timestamps by their value. They're
parsed as RFC3339 timestamps, so validation will still fail if the
format is totally wrong, but this affords some leniency in the
formatting.

[1] From https://golang.org/pkg/time/#Time.MarshalJSON: "The time is a
quoted string in RFC 3339 format, with sub-second precision added
if present."

events/validators.go

@@ -18,6 +18,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"reflect"
+	"time"
 
 	cloudevents "github.com/cloudevents/sdk-go/v2"
 	"github.com/google/go-cmp/cmp"
@@ -94,13 +95,13 @@ func validateLegacyEvent(name string, gotBytes, wantBytes []byte) *ValidationInf
 	got := make(map[string]interface{})
 	err := json.Unmarshal(gotBytes, &got)
 	if err != nil {
-		vi.Errs = append(vi.Errs, fmt.Errorf("unmarshalling function-received version of legacy event %q: %v", name, err))
+		vi.Errs = append(vi.Errs, fmt.Errorf("unmarshalling received legacy event %q: %v", name, err))
 	}
 
 	want := make(map[string]interface{})
 	err = json.Unmarshal(wantBytes, &want)
 	if err != nil {
-		vi.Errs = append(vi.Errs, fmt.Errorf("unmarshalling expected contents of legacy event %q: %v", name, err))
+		vi.Errs = append(vi.Errs, fmt.Errorf("unmarshalling expected legacy event %q: %v", name, err))
 	}
 
 	// If there were issues extracting the data, bail early.
@@ -117,6 +118,16 @@ func validateLegacyEvent(name string, gotBytes, wantBytes []byte) *ValidationInf
 		gotValue  interface{}
 		wantValue interface{}
 	}
+	gotTimestamp, err := time.Parse(time.RFC3339, gotContext["timestamp"].(string))
+	if err != nil {
+		vi.Errs = append(vi.Errs, fmt.Errorf("parsing timestamp of received legacy event: %v", err))
+		return vi
+	}
+	wantTimestamp, err := time.Parse(time.RFC3339, wantContext["timestamp"].(string))
+	if err != nil {
+		vi.Errs = append(vi.Errs, fmt.Errorf("parsing timestamp of expected legacy event: %v", err))
+		return vi
+	}
 	fields := []eventFields{
 		{
 			name:      "ID",
@@ -130,8 +141,8 @@ func validateLegacyEvent(name string, gotBytes, wantBytes []byte) *ValidationInf
 		},
 		{
 			name:      "timestamp",
-			gotValue:  gotContext["timestamp"],
-			wantValue: wantContext["timestamp"],
+			gotValue:  gotTimestamp,
+			wantValue: wantTimestamp,
 		},
 		{
 			name:      "resource",