Fix a bug in std.Thread.Condition and add a basic Condition test. (#10538)

* Fix FUTEX usage in std.Thread.Condition - It was using an old name.

lib/std/Thread.zig

@@ -1151,3 +1151,27 @@ test "Thread.detach" {
     event.wait();
     try std.testing.expectEqual(value, 1);
 }
+
+fn testWaitForSignal(mutex: *Mutex, cond: *Condition) void {
+    mutex.lock();
+    defer mutex.unlock();
+    cond.signal();
+    cond.wait(mutex);
+}
+
+test "Condition.signal" {
+    if (builtin.single_threaded) return error.SkipZigTest;
+
+    var mutex = Mutex{};
+    var cond = Condition{};
+
+    var thread: Thread = undefined;
+    {
+        mutex.lock();
+        defer mutex.unlock();
+        thread = try Thread.spawn(.{}, testWaitForSignal, .{ &mutex, &cond });
+        cond.wait(&mutex);
+        cond.signal();
+    }
+    thread.join();
+}