Add . to perl path for all perl tests

* tests/*.px: Add -I . to the shebang

This allows perl test to be run from tests/ directory, e.g. via
  ./Test--post-file.px

tests/Test-https-clientcert.px

@@ -1,4 +1,4 @@
-#!/usr/bin/env perl
+#!/usr/bin/env -S perl -I .
 
 use strict;
 use warnings;