Unit tests fails when running them inside a Kubernetes cluster

**Describe the bug**
Arguable a somewhat special edge case, but the [introduction](f53d038a4ab908b1eb57da39117cfc34d6163c7c) of `DefaultOIDCDiscoveryBaseURL` with its default value `https://kubernetes.default.svc` leads to a panic when trying to run the unit tests in a CI that runs in Kubernetes. I suspect that the initialization picks up that endpoint and can resolve it in-cluster and then tries to use it. Reason why I suspect this is that if you replace `kubernetes.default.svc` with something that cannot be resolved in-cluster, the test case does pass.

```
panic: test timed out after 10m0s
running tests:
TestReceiver (10m0s)
TestReceiver/Dispatch_succeeded_-_Source_with_type (1m0s)

goroutine 244 [select]:
net/http.(*Transport).getConn(0x35a79e0, 0xc00029d810, {{}, 0x0, {0xc00037e4c0, 0x5}, {0xc00005b580, 0x1a}, 0x0})
/usr/lib/golang/src/net/http/transport.go:1468 +0x4bd
net/http.(*Transport).roundTrip(0x35a79e0, 0xc0003d23c0)
/usr/lib/golang/src/net/http/transport.go:633 +0xb54
net/http.(*Transport).RoundTrip(0x0?, 0x2453e60?)
/usr/lib/golang/src/net/http/roundtrip.go:30 +0x13
net/http.send(0xc0003d23c0, {0x2453e60, 0x35a79e0}, {0xc0001ed301?, 0x41752b?, 0x0?})
/usr/lib/golang/src/net/http/client.go:259 +0x5e4
net/http.(*Client).send(0x362db80, 0xc0003d23c0, {0xc00037e4de?, 0x21?, 0x0?})
/usr/lib/golang/src/net/http/client.go:180 +0x98
net/http.(*Client).do(0x362db80, 0xc0003d23c0)
/usr/lib/golang/src/net/http/client.go:725 +0x8bc
net/http.(*Client).Do(...)
/usr/lib/golang/src/net/http/client.go:590
net/http.(*Client).Get(0x362db80, {0xc00037e4c0?, 0x2173689?})
/usr/lib/golang/src/net/http/client.go:487 +0x5f
[knative.dev/eventing/pkg/auth.(*Verifier).getKubernetesOIDCDiscovery](http://knative.dev/eventing/pkg/auth.(*Verifier).getKubernetesOIDCDiscovery)(0xc00029cbe0?, 0xc0001f2bc0?, 0x362db80)
/workspace/app/repo/src/eventing/pkg/auth/verifier.go:316 +0xb2
[knative.dev/eventing/pkg/auth.(*Verifier).initOIDCProvider](http://knative.dev/eventing/pkg/auth.(*Verifier).initOIDCProvider)(0xc00029cbe0, {0x2476780, 0xc00020a210}, 0xc00020a4b0)
/workspace/app/repo/src/eventing/pkg/auth/verifier.go:248 +0xd2
[knative.dev/eventing/pkg/auth.NewVerifier](http://knative.dev/eventing/pkg/auth.NewVerifier)({0x2476780, 0xc00020a210}, {0x245f660, 0xc00036a890}, {0x245d0b8, 0xc0000a4740}, {0x245b900, 0xc000128280})
/workspace/app/repo/src/eventing/pkg/auth/verifier.go:88 +0x5b5
[knative.dev/eventing/pkg/broker/filter.TestReceiver.func4(0xc0001476c0)](http://knative.dev/eventing/pkg/broker/filter.TestReceiver.func4(0xc0001476c0))
/workspace/app/repo/src/eventing/pkg/broker/filter/filter_handler_test.go:456 +0x5a5
testing.tRunner(0xc0001476c0, 0xc00036a590)
/usr/lib/golang/src/testing/testing.go:1690 +0xf4
created by testing.(*T).Run in goroutine 24
/usr/lib/golang/src/testing/testing.go:1743 +0x390

FAIL [knative.dev/eventing/pkg/broker/filter](http://knative.dev/eventing/pkg/broker/filter) 600.063s
panic: test timed out after 10m0s
running tests:
TestHandler_ServeHTTP (10m0s)
TestHandler_ServeHTTP/malformed_request_URI (1m0s)
```

**Expected behavior**
Unit tests do run also in a Kubernetes provided containerized environment, if possible.

**To Reproduce**
Run unit tests inside a Kubernetes pod, e.g. Tekton setup.

**Knative release version**
[v0.43.0](https://github.com/knative/eventing/releases/tag/v0.43.0)
[knative-v1.16.0](https://github.com/knative/eventing/releases/tag/knative-v1.16.0)

**Additional context**
n/a