fix incorrect usage of errors.Is

errors.Is is supposed to used for equality of errors, not for type
assertions. That's what errors.As is there for.

integrationtests/self/mitm_test.go

@@ -371,7 +371,8 @@ var _ = Describe("MITM test", func() {
 					}
 					err := runTest(delayCb)
 					Expect(err).To(HaveOccurred())
-					Expect(err).To(MatchError(&quic.VersionNegotiationError{}))
+					vnErr := &quic.VersionNegotiationError{}
+					Expect(errors.As(err, &vnErr)).To(BeTrue())
 				})
 
 				// times out, because client doesn't accept subsequent real retry packets from server

integrationtests/self/stateless_reset_test.go

@@ -2,6 +2,7 @@ package self_test
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"math/rand"
 	"net"
@@ -99,7 +100,8 @@ var _ = Describe("Stateless Resets", func() {
 				_, serr = str.Read([]byte{0})
 			}
 			Expect(serr).To(HaveOccurred())
-			Expect(serr).To(MatchError(&quic.StatelessResetError{}))
+			statelessResetErr := &quic.StatelessResetError{}
+			Expect(errors.As(serr, &statelessResetErr)).To(BeTrue())
 			Expect(ln2.Close()).To(Succeed())
 			Eventually(acceptStopped).Should(BeClosed())
 		})