Add Set-Cookie parsing

[karlseguin]

This code isn't hooked into anything. It's only a Set-Cookie response header parser (which includes a DateTime parser).

Next step:

1. Save them in the browser
2. When sending a request, figure out which cookies should be sent
3. Include them in the request

I added a testing.zig. I needed a function to assert an integer was within a delta:

test "DateTime: now" {
    const dt = DateTime.now();
    try testing.expectDelta(std.time.microTimestamp(), dt.micros, 1000);
}

std.testing only has this for floats.

It also expands expectEqual to work against strings. I think is nice on its own, but it's particularly useful when testing against an anytype.

fn expectCookieAttributes(attributes: anytype, actual: Cookie) !void {
   inline for (@typeInfo(@TypeOf(attributes)).Struct.fields) |field| {
      testing.expectEqual(@field(anytype, field.name), @field(actual, field.name))
   }
}

This doesn't work with std.expectEqual if any of the values are strings.