test: fix SAML IdP unit tests (#11270)

# Which Problems Are Solved

Similar to https://github.com/zitadel/zitadel/pull/11241, there are
additional unit tests where certificates expired.

# How the Problems Are Solved

The tests use dedicated combinations of SAML metadata, certificates and
responses and also already use the `TimeNow` function from the
`crewjam/saml` package to overwrite necessary time validations. This PR
now additionally sets the `Clock` variable in the same package, which is
used in XML validations like the signing.

# Additional Changes

None

# Additional Context

- relates to #11241
- requires backport to at least v4.x

internal/idp/providers/saml/session_test.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/crewjam/saml"
+	dsig "github.com/russellhaering/goxmldsig"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
@@ -377,6 +378,7 @@ func TestSession_FetchUser(t *testing.T) {
 
 			// set to time of response for validation
 			saml.TimeNow = tt.fields.timeNow
+			saml.Clock = dsig.NewFakeClockAt(tt.fields.timeNow())
 
 			user, err := session.FetchUser(context.Background())
 			if tt.want.err != nil && !errors.Is(err, tt.want.err) {