Swap out importing jetzig.zig for explicit files containing tests

Latest Zig nightly now hits a panic, probably we have a cyclic reference
in the tree. We should fix this ASAP otherwise we risk adding tests
without running them, but for now this should get builds passing again
with latest Zig.

src/tests.zig

@@ -1,5 +1,6 @@
 test {
-    _ = @import("jetzig.zig");
     _ = @import("jetzig/http/Query.zig");
+    _ = @import("jetzig/http/Headers.zig");
+    _ = @import("jetzig/http/Cookies.zig");
     @import("std").testing.refAllDeclsRecursive(@This());
 }