simulator: Add basic test for the clock

src/simulator.zig

@@ -96,3 +96,7 @@ fn getExecutableEntryPoint(self: *Simulator, name: []const u8) ?EntryPoint {
     }
     return null;
 }
+
+test {
+    _ = @import("test_simulator.zig");
+}

src/test_simulator.zig

@@ -0,0 +1,28 @@
+const std = @import("std");
+
+const Simulator = @import("simulator.zig");
+
+const Io = std.Io;
+const Clock = Io.Clock;
+
+fn testClockGrowsMonotonically(init: std.process.Init) anyerror!void {
+    const io = init.io;
+
+    const t1 = Clock.boot.now(io);
+    const t2 = Clock.boot.now(io);
+    try std.testing.expect(t1.nanoseconds <= t2.nanoseconds);
+
+    const t3 = Clock.boot.now(io);
+    try std.testing.expect(t2.nanoseconds <= t3.nanoseconds);
+}
+
+test "clock grows monotonically" {
+    var sim: Simulator = undefined;
+    sim.init(std.heap.page_allocator, std.Options.debug_io);
+    defer sim.deinit();
+
+    try sim.addExecutable("program_a", testClockGrowsMonotonically);
+    try sim.spawn("program_a", .{});
+
+    while (sim.scheduleOne()) {}
+}