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/url/query.zig

@@ -62,6 +62,7 @@ pub const Values = struct {
     // append by taking the ownership of the key and the value
     fn appendOwned(self: *Values, k: []const u8, v: []const u8) !void {
         if (self.map.getPtr(k)) |list| {
+            self.alloc.free(k);
             return try list.append(self.alloc, v);
         }