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
Kubernetes-commit: 457df1cf9817e29381609fbf2d85854a6b6a52ac
