test handling of command exec errors

This adds a test for common command exec errors when checking out
repositories. The test checks if the correct error is thrown
(CommandExecError) instead of failing silently or with an
unspecific exception.

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

tests/test_commands/test-invalid.yml

@@ -0,0 +1,7 @@
+header:
+  version: 8
+
+repos:
+  kas_invalid:
+    url: https://example.com/kas.git
+    refspec: this-ref-is-invalid