Fixed wrong plugin options behaviour

While working on MR !230 I noticed that the options read I entered into several plugins could not be read after reading the image.

**The patch fixes problems reading options in plugins and adds option checking in the readtest.cpp.**

In particular, the reading test does the following additional actions:
- reads options before reading the image;
- compare the options read with the options returned by the reader after reading the image;
- compares the format and size of the returned image with the format and size returned by the reader.

src/imageformats/tga_p.h

@@ -9,7 +9,9 @@
 #define KIMG_TGA_P_H
 
 #include <QImageIOPlugin>
+#include <QScopedPointer>
 
+class TGAHandlerPrivate;
 class TGAHandler : public QImageIOHandler
 {
 public:
@@ -23,6 +25,9 @@ public:
     QVariant option(QImageIOHandler::ImageOption option) const override;
 
     static bool canRead(QIODevice *device);
+
+private:
+    const QScopedPointer<TGAHandlerPrivate> d;
 };
 
 class TGAPlugin : public QImageIOPlugin