tests: move to t, allow yath test from root

By moving the tests subdirectory to t, we gain the ability to run `yath
test` with no arguments from inside `nix develop` in the root of the
the repo.

(`nix develop` is necessary in order to set the proper env vars for
`yath` to find our test libraries.)

t/test.pl

@@ -0,0 +1,24 @@
+#!/usr/bin/env perl
+# HARNESS-NO-PRELOAD
+# HARNESS-CAT-LONG
+# THIS IS A GENERATED YATH RUNNER TEST
+use strict;
+use warnings;
+
+use lib 'lib';
+BEGIN {
+    use File::Which qw(which);
+    $App::Yath::Script::SCRIPT = which 'yath';
+}
+use App::Yath::Util qw/find_yath/;
+
+system($^X, find_yath(), '-D', 'test', '--default-search' => './', @ARGV);
+my $exit = $?;
+
+# This makes sure it works with prove.
+print "1..1\n";
+print "not " if $exit;
+print "ok 1 - Passed tests when run by yath\n";
+print STDERR "yath exited with $exit" if $exit;
+
+exit($exit ? 255 : 0);