message: play around with unit testing

I think I am probably going to move unit tests to a separate
directory/file structure. This will allow me to add a bunch of utility
functions that don't get analyzed for the library compilation and also
avoid testing-only imports in the main modules.

src/message.zig

@@ -113,3 +113,63 @@ pub const Message = opaque {
 // NATS_EXTERN const char* stanMsg_GetData(const stanMsg *msg);
 // NATS_EXTERN int stanMsg_GetDataLength(const stanMsg *msg);
 // NATS_EXTERN void stanMsg_Destroy(stanMsg *msg);
+
+test "message: create message" {
+    const subject = "hello";
+    const reply = "reply";
+    const data = "world";
+
+    const nats = @import("./nats.zig");
+
+    // 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 Message.create(subject, reply, data);
+    defer message.destroy();
+
+    const message2 = try Message.create(subject, null, data);
+    defer message2.destroy();
+
+    const message3 = try Message.create(subject, data, null);
+    defer message3.destroy();
+
+    const message4 = try Message.create(subject, null, null);
+    defer message4.destroy();
+}
+
+test "message: get subject" {
+    const nats = @import("./nats.zig");
+
+    try nats.init(-1);
+    defer nats.deinit();
+
+    const subject = "hello";
+    const message = try Message.create(subject, null, null);
+    defer message.destroy();
+
+    const received = message.getSubject();
+    try std.testing.expectEqualStrings(subject, received);
+}
+
+test "message: get reply" {
+    const nats = @import("./nats.zig");
+
+    try nats.init(-1);
+    defer nats.deinit();
+
+    const subject = "hello";
+    const reply = "reply";
+    const message = try 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 Message.create(subject, null, null);
+    defer message2.destroy();
+
+    const received2 = message2.getReply();
+    try std.testing.expect(received2 == null);
+}

src/nats.zig

@@ -150,6 +150,10 @@ pub const Statistics = opaque {
     }
 };
 
+test {
+    std.testing.refAllDecls(@This());
+}
+
 // NATS_EXTERN natsStatus nats_Sign(const char *encodedSeed, const char *input, unsigned char **signature, int *signatureLength);
 
 // NATS_EXTERN natsStatus natsOptions_Create(natsOptions **newOpts);