Your solution will probably work too, but I've found a simpler solution - less changes in current code.

I just iterate over all tests on my own using TAP::Parser and now top-level TAP::Harness is happy.

#!/usr/bin/perl
use strict;
use warnings;
use TAP::Harness;
use TAP::Parser;
use Test::More 'no_plan';

my $tap = TAP::Harness->new(
    {   'color'     => 1,
        'verbosity' => 1,
        'lib'       => ['lib', 'blib/lib', 'blib/arch'],
        'timer'     => 1
    }
);

my @tests = (glob('t/unit/**/*.t'), glob('t/unit/*.t'));

foreach my $test (@tests) {
    my $parser = TAP::Parser->new({source => \$test});
    while (my $result = $parser->next) {
        next unless $result->is_test;
        ok($result->is_ok, "[$test] " . $result->as_string);
    }
}

ok(1, "Unit tests run");
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