Update tests to work with @inc that does not contain '.'

Bug report: https://rt.cpan.org/Ticket/Display.html?id=121458

Perl 5.25.11 removes '.' from the default @inc, and this breaks
`require "test_data/gen_test_data.pl"` due to not being able to assume
./test_data/gen_test_data.pl is in the @inc load path, and due to
`require` treating paths without leading "/" or "./" as search paths.

This change implements the first solution suggested by the reporter of
the bug: Re-organise test dependencies into a dedicated directory such
as `t/lib` and load that path into @inc with `use lib 't/lib'`.

t/frconvert.t

@@ -1,8 +1,9 @@
 #!/usr/bin/env perl -w
 use strict;
 use Test;
+use lib 't/lib';
 BEGIN { plan tests => 1 }
-require "test_data/gen_test_data.pl";
+require "gen_test_data.pl";
 
 gen("foo");
 use Locale::gettext;

t/jaconvert.t

@@ -1,8 +1,9 @@
 #!/usr/bin/env perl -w
 use strict;
 use Test;
+use lib 't/lib';
 BEGIN { plan tests => 1 }
-require "test_data/gen_test_data.pl";
+require "gen_test_data.pl";
 
 gen("jaeuc");
 use Locale::gettext;

t/raw.t

@@ -1,8 +1,9 @@
 #!/usr/bin/env perl -w
 use strict;
 use Test;
+use lib 't/lib';
 BEGIN { plan tests => 1 }
-require "test_data/gen_test_data.pl";
+require "gen_test_data.pl";
 
 gen("foo");
 use Locale::gettext;