Add a new unittest build step

Preserves all existing behavior (i.e. make test and zig build test are not
changed in any way).

The new 'unittest' only runs unit tests and is fast to build. It takes ~1.7 to
build unittest, vs ~11.09 to build test. This is really the main goal, and
hopefully any unit test which are (a) fast and (b) don't impact build times
will be run here.

The test runner is based on:
https://gist.github.com/karlseguin/c6bea5b35e4e8d26af6f81c22cb5d76b

It allow filtering, i.e. `make unittest F="parse query dup"`.

'unittest' does memory leak detection when tests use std.testing.allocator.

Fixed a memory leak in url/query which was detected/reported with by the new
'unittest'.

In order to avoid having 3 src/test_xyx.zig files, I merged the existing
test_runner.zig and run_tests.zig into a single main_tests.zig. (this change
is superfluous, but I thought it was cleaner this way. Happy to revert this).

src/test_runner.zig

@@ -1,29 +0,0 @@
-// Copyright (C) 2023-2024  Lightpanda (Selecy SAS)
-//
-// Francis Bouvier <francis@lightpanda.io>
-// Pierre Tachoire <pierre@lightpanda.io>
-//
-// This program is free software: you can redistribute it and/or modify
-// it under the terms of the GNU Affero General Public License as
-// published by the Free Software Foundation, either version 3 of the
-// License, or (at your option) any later version.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU Affero General Public License for more details.
-//
-// You should have received a copy of the GNU Affero General Public License
-// along with this program.  If not, see <https://www.gnu.org/licenses/>.
-
-const std = @import("std");
-
-const tests = @import("run_tests.zig");
-
-pub const Types = tests.Types;
-pub const UserContext = tests.UserContext;
-pub const IO = tests.IO;
-
-pub fn main() !void {
-    try tests.main();
-}