test(xcat-dep): hold the riscv64 cell to the builder's package set

The assertion fails against the previous manifest, naming perl-HTML-Form as
the package the cell lacked. It reads the set from the builder, so a package
added to the builder later fails here until the cell names it.

t/riscv64_perl_cell.t

@@ -0,0 +1,33 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+
+use FindBin qw($RealBin);
+use Test::More;
+
+use lib $RealBin, "$RealBin/..";
+use MockBuildUtils qw(read_manifest);
+
+# EL10 riscv64 has no EPEL, so every perl package the builder exists to supply must be pinned in
+# the riscv64 cell: an omission there is not filled in by any other repository. perl-HTML-Form
+# was missing, and perl-xCAT hard-requires perl(HTML::Form), so `dnf install xCAT` could not
+# resolve on riscv64 at all. The set comes from the builder itself, so a package added to it later
+# fails here until the cell names it.
+
+my $builder = "$RealBin/../mockbuild-perl-packages.pl";
+plan skip_all => 'mockbuild-perl-packages.pl not found' unless -f $builder;
+plan skip_all => 'Parallel::ForkManager is not available' unless eval { require Parallel::ForkManager; 1 };
+
+my @built = grep { length } split /\n/, `perl -I "$RealBin/.." -I "$RealBin/../lib" "$builder" --epel-gap --list-packages 2>/dev/null`;
+is( $?, 0, 'the builder lists the packages an --epel-gap run would build' );
+cmp_ok( scalar(@built), '>=', 10, '... and the list is the real one, not an empty run' );
+
+my %m = read_manifest("$RealBin/../packages-manifest.conf");
+my $cell = $m{'rocky-10-riscv64-xcat'};
+ok( $cell, 'the manifest has the riscv64 cell' );
+
+my @unpinned = grep { !defined $cell->{$_} } @built;
+is_deeply( \@unpinned, [], 'every package the builder supplies is pinned in the riscv64 cell' )
+  or diag( "missing from [rocky-10-riscv64-xcat]: @unpinned" );
+
+done_testing;