Add tests against reading config files (#2682)

* Add tests

* Use test tables

* Rename test config to better show its purpose

* Use temporary environment for testing

* Check also expansion with curly braces

---------

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

cmd/testdata/env.actrc

@@ -0,0 +1,2 @@
+--artifact-server-path $FAKEPWD/.artifacts
+--env FOO=prefix/${FOO}/suffix

cmd/testdata/simple.actrc

@@ -0,0 +1,2 @@
+--container-architecture=linux/amd64
+--action-offline-mode

cmd/testdata/split.actrc

@@ -0,0 +1 @@
+--container-options --volume /foo:/bar --volume /baz:/qux --volume /tmp:/tmp