add test for resolve-env option in dump plugin

This patch adds a test for the --resolve-env option in the dump plugin.
Both cases resolve and non-resolve are tested, each for yaml and json
output format.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_repo_includes/test.yml

@@ -4,6 +4,9 @@ header:
     - repo: subrepo
       file: test.yml
 
+env:
+  TESTVAR_FOO: "BAR"
+
 repos:
   this: