Add emit-on-change tracker for the Windows polling sensor

internal/evidence/foreground_tracker_test.go

@@ -0,0 +1,29 @@
+package evidence
+
+import "testing"
+
+func TestForegroundTrackerChanged(t *testing.T) {
+	var tr foregroundTracker
+
+	if !tr.changed(true, 100, "A") {
+		t.Fatal("first observation should always report changed")
+	}
+	if tr.changed(true, 100, "A") {
+		t.Error("identical observation should not report changed")
+	}
+	if !tr.changed(true, 100, "B") {
+		t.Error("title change on same hwnd should report changed")
+	}
+	if !tr.changed(true, 200, "B") {
+		t.Error("hwnd change should report changed")
+	}
+	if !tr.changed(false, 0, "") {
+		t.Error("transition to unavailable should report changed")
+	}
+	if tr.changed(false, 0, "") {
+		t.Error("repeated unavailable should not report changed")
+	}
+	if !tr.changed(true, 200, "B") {
+		t.Error("transition back to available should report changed")
+	}
+}