tests: add basic connection test

I created a utility class to spawn a NATS server. This assumes it is
installed and in PATH, which should be easy enough to accomplish for a
CI environment. The current approach will not work with parallel
tests, but that's not a practical reality, so this route should be
fine for the time being. It might be nice to spawn a default server at
the beginning of testing and tear it down at the end, to save waiting
for the startup/shutdown time in many individual tests. This makes me
wonder: if I initialize a server in the beginning of the `test` block
in main that imports the other modules, does that scope stay live
while the "child" tests are running? My default guess would be
probably not, but that would be very convenient, so I might try it out
and see.

tests/connection.zig

@@ -0,0 +1,18 @@
+const std = @import("std");
+
+const nats = @import("nats");
+
+const util = @import("./util.zig");
+
+test "nats.Connection.connectTo" {
+    var server = try util.TestServer.launch(.{});
+    defer server.stop();
+
+    {
+        try nats.init(nats.default_spin_count);
+        defer nats.deinit();
+
+        const connection = try nats.Connection.connectTo(nats.default_server_url);
+        defer connection.destroy();
+    }
+}

tests/main.zig

@@ -1,4 +1,5 @@
 test {
     _ = @import("./nats.zig");
+    _ = @import("./connection.zig");
     _ = @import("./message.zig");
 }

tests/util.zig

@@ -0,0 +1,73 @@
+const std = @import("std");
+
+const TestLaunchError = error{
+    NoLaunchStringFound,
+};
+
+pub const TestServer = struct {
+    process: std.ChildProcess,
+
+    pub const LaunchOptions = struct {
+        executable: []const u8 = "nats-server",
+        port: u16 = 4222,
+        auth: union(enum) {
+            none: void,
+            token: []const u8,
+            password: struct { user: []const u8, pass: []const u8 },
+        } = .none,
+        allocator: std.mem.Allocator = std.testing.allocator,
+
+        fn argLen(self: LaunchOptions) usize {
+            // executable, -a, 127.0.0.1, -p, 4222
+            const base_len: usize = 5;
+            return base_len + switch (self.auth) {
+                .none => @as(usize, 0),
+                .token => @as(usize, 2),
+                .password => @as(usize, 4),
+            };
+        }
+    };
+
+    pub fn launch(options: LaunchOptions) !TestServer {
+        // const allocator = std.testing.allocator;
+        var portbuf = [_]u8{0} ** 5;
+        const strport = try std.fmt.bufPrint(&portbuf, "{d}", .{options.port});
+
+        const argsbuf: [9][]const u8 = blk: {
+            const executable: [1][]const u8 = .{options.executable};
+            const listen: [2][]const u8 = .{ "-a", "127.0.0.1" };
+            const port: [2][]const u8 = .{ "-p", strport };
+            const auth: [4][]const u8 = switch (options.auth) {
+                .none => .{""} ** 4,
+                .token => |tok| .{ "--auth", tok, "", "" },
+                .password => |auth| .{ "--user", auth.user, "--password", auth.pass },
+            };
+
+            break :blk executable ++ listen ++ port ++ auth;
+        };
+
+        const args = argsbuf[0..options.argLen()];
+
+        var child = std.ChildProcess.init(args, options.allocator);
+        child.stdin_behavior = .Ignore;
+        child.stdout_behavior = .Pipe;
+        child.stderr_behavior = .Pipe;
+
+        try child.spawn();
+        var poller = std.io.poll(options.allocator, enum { stderr }, .{ .stderr = child.stderr.? });
+        defer poller.deinit();
+
+        while (try poller.poll()) {
+            if (std.mem.indexOf(u8, poller.fifo(.stderr).buf, "[INF] Server is ready")) |_| {
+                return .{ .process = child };
+            }
+        }
+
+        _ = try child.kill();
+        return error.NoLaunchStringFound;
+    }
+
+    pub fn stop(self: *TestServer) void {
+        _ = self.process.kill() catch return;
+    }
+};