Add unit test for rule with invalid output.

Add the ability to check falco's return code with exit_status and to
generally match stderr with stderr_contains in a test.

Use those to create a test that has an invalid output expression using
%not_a_real_field. It expects falco to exit with 1 and the output to
contain a message about the invalid output.

test/rules/invalid_rule_output.yaml

@@ -0,0 +1,5 @@
+- rule: rule_with_invalid_output
+  desc: A rule with an invalid output field
+  condition: evt.type=open
+  output: "An open was seen %not_a_real_field"
+  priority: WARNING