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();
+}

lib/std/Thread/Condition.zig

@@ -106,7 +106,7 @@ pub const AtomicCondition = struct {
                     .linux => {
                         switch (linux.getErrno(linux.futex_wait(
                             &cond.futex,
-                            linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAIT,
+                            linux.FUTEX.PRIVATE_FLAG | linux.FUTEX.WAIT,
                             0,
                             null,
                         ))) {
@@ -128,7 +128,7 @@ pub const AtomicCondition = struct {
                 .linux => {
                     switch (linux.getErrno(linux.futex_wake(
                         &cond.futex,
-                        linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAKE,
+                        linux.FUTEX.PRIVATE_FLAG | linux.FUTEX.WAKE,
                         1,
                     ))) {
                         .SUCCESS => {},