[tests] add test for 11990 (#11992)

* [tests] add test for 11990

* [tests] add test for F64 too

tests/unit/src/unit/issues/Issue11990.hx

@@ -0,0 +1,14 @@
+package unit.issues;
+
+class Issue11990 extends unit.Test {
+	public function test() {
+		#if (cpp || jvm || hl)
+		var x:Single = std.Math.NaN;
+		eq(true, x != x);
+		var x:Float = std.Math.NaN;
+		eq(true, x != x);
+		#else
+		noAssert();
+		#end
+	}
+}