t4018: convert perl pattern tests to the new infrastructure

There is one subtlety: The old test case 'perl pattern gets full line of
POD header' does not have its own new test case, but the feature is
tested nevertheless by placing the RIGHT tag at the end of the expected
hunk header in t4018/perl-skip-sub-in-pod.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/perl-skip-forward-decl

@@ -0,0 +1,10 @@
+package RIGHT;
+
+use strict;
+use warnings;
+use parent qw(Exporter);
+our @EXPORT_OK = qw(round finalround);
+
+sub other; # forward declaration
+
+# ChangeMe