REGTESTS: Add reg tests for the HTTP return action

2 reg tests have been added to ensure the HTTP return action is functionnal. A
reg test is about returning error files. The other one is about returning
default responses and responses based on string or file payloads.
diff --git a/reg-tests/http-rules/lf-file.txt b/reg-tests/http-rules/lf-file.txt
new file mode 100644
index 0000000..7fda1d4
--- /dev/null
+++ b/reg-tests/http-rules/lf-file.txt
@@ -0,0 +1 @@
+path is %[url]