Here's the guts of the Module::Build extension I use in Jellybean.

sub ACTION_test
{
    my $self = shift;
    my @files;
    find( sub { /\.t\z/ and push @files, $File::Find::name }, 't' );
    $self->SUPER::ACTION_test( 'test_files=' . join(' ', @files ) );
}
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There's no reason you couldn't do something similar in your Makefile.PL, though you might run into command-line length limits.