tests: start building tests in separate directory

This approach seems nice, and as a bonus it makes it easier to run the
tests through the module interface rather than by importing the
sources directly, which I think is a good dog food approach.

tests/main.zig

@@ -0,0 +1,5 @@
+const std = @import("std");
+
+test {
+    _ = @import("./message.zig");
+}

tests/message.zig

@@ -0,0 +1,59 @@
+const std = @import("std");
+const nats = @import("nats");
+
+// const nats = @import("../src/nats.zig");
+// const message = @import("../src/message.zig");
+
+test "message: create message" {
+    const subject = "hello";
+    const reply = "reply";
+    const data = "world";
+
+    // have to initialize the library so the reference counter can correctly destroy
+    // objects, otherwise we segfault on trying to free the memory.
+    try nats.init(-1);
+    defer nats.deinit();
+
+    const message = try nats.Message.create(subject, reply, data);
+    defer message.destroy();
+
+    const message2 = try nats.Message.create(subject, null, data);
+    defer message2.destroy();
+
+    const message3 = try nats.Message.create(subject, data, null);
+    defer message3.destroy();
+
+    const message4 = try nats.Message.create(subject, null, null);
+    defer message4.destroy();
+}
+
+test "message: get subject" {
+    try nats.init(-1);
+    defer nats.deinit();
+
+    const subject = "hello";
+    const message = try nats.Message.create(subject, null, null);
+    defer message.destroy();
+
+    const received = message.getSubject();
+    try std.testing.expectEqualStrings(subject, received);
+}
+
+test "message: get reply" {
+    try nats.init(-1);
+    defer nats.deinit();
+
+    const subject = "hello";
+    const reply = "reply";
+    const message = try nats.Message.create(subject, reply, null);
+    defer message.destroy();
+
+    const received = message.getReply() orelse return error.TestUnexpectedResult;
+    try std.testing.expectEqualStrings(reply, received);
+
+    const message2 = try nats.Message.create(subject, null, null);
+    defer message2.destroy();
+
+    const received2 = message2.getReply();
+    try std.testing.expect(received2 == null);
+}