ci cleanup: make sure to remove test KUBECONFIGs

Calling os.Exit() skipped the

    `defer os.RemoveAll(tmp)`

above and thus dangling files & dirs would appear in their $TMPDIR.

Since go1.15 we can safely remove os.Exit() from TestMain() function.
As this exact issue was common enough so that go1.15 changed[1] the API of
TestMain to no longer require os.Exit to be called.

Reproducer:

    $ (cd staging/src/k8s.io/client-go; go test ./tools/clientcmd)
    $ ls -d /tmp/testkubeconfig*
    /tmp/testkubeconfig1015943687

[1]: https://go-review.googlesource.com/c/go/+/219639

staging/src/k8s.io/client-go/tools/clientcmd/main_test.go

@@ -29,5 +29,5 @@ func TestMain(m *testing.M) {
 	}
 	defer os.RemoveAll(tmp)
 	os.Setenv("KUBECONFIG", filepath.Join(tmp, "dummy-nonexistent-kubeconfig"))
-	os.Exit(m.Run())
+	m.Run()
 }