PropertyMap tests went through SetPermissions, which looks up the `dokku` user/group; the fresh `golang:1.26.2` container used by `go-test-plugin-in-docker` has neither, so the chown failed with `unknown group dokku`. Override `DOKKU_SYSTEM_USER` and `DOKKU_SYSTEM_GROUP` to `root` in the property and chart-migration test setups so the chown targets an identity that exists everywhere. The config tests also called `plugn trigger post-config-update` without `PLUGIN_CORE_AVAILABLE_PATH` set, so the hook tried to source `/common/functions` and logged a noisy warning on every test; point the env var at the canonical `/var/lib/dokku/core-plugins/available` location.
- config_test.go: create app dir at DOKKU_ROOT for VerifyAppName and
ensure global config parent directory exists
- migrateGlobalEnv: set dokku:dokku ownership on global ENV file after
writing so deploy processes running as dokku can read it
The io/ioutil package has been deprecated as of Go 1.16 [1]. This commit
replaces the existing io/ioutil functions with their new definitions in
io and os packages.
[1]: https://golang.org/doc/go1.16#ioutil
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>