tst_QLibrary: add a cleanup() method to unload left-overs

QLibrary intentionally does not unload on destruction, so failing tests
may leave libraries already loaded and cause further tests to fail
because of that. So add a cleanup() method to unload everything we may
have loaded.

Note that QLibrary::unload() sets its state to NotLoaded after one
successful call, so we must recreate the object in case it had been
load()ed multiple times.

Change-Id: I12a088d1ae424825abd3fffd171d133c678f910a
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 636dbe604596b2860e4f6d9b159fc017dcd2f66b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/corelib/plugin/qlibrary/tst_qlibrary.cpp

@@ -81,6 +81,7 @@ enum QLibraryOperation {
     QString directory;
 private slots:
     void initTestCase();
+    void cleanup();
 
     void load();
     void load_data();
@@ -124,6 +125,38 @@ void tst_QLibrary::initTestCase()
 #endif
 }
 
+void tst_QLibrary::cleanup()
+{
+    // unload the libraries, if they are still loaded after the test ended
+    // (probably in a failure)
+
+    static struct {
+        QString name;
+        int version = -1;
+    } libs[] = {
+        { directory + "/mylib" },
+        { directory + "/mylib", 1 },
+        { directory + "/mylib", 2 },
+        { sys_qualifiedLibraryName("mylib") },
+
+        // stuff that load_data() succeeds with
+        { directory + "/" PREFIX "mylib" },
+        { directory + "/" PREFIX "mylib" SUFFIX },
+#if defined(Q_OS_WIN32)
+        { directory + "/mylib.dl2" },
+        { directory + "/system.qt.test.mylib.dll" },
+#elif !defined(Q_OS_ANDROID)
+        // .so even on macOS
+        { directory + "/libmylib.so2" },
+        { directory + "/system.qt.test.mylib.so" },
+#endif
+
+    };
+    for (const auto &entry : libs) {
+        do {} while (QLibrary(entry.name, entry.version).unload());
+    }
+}
+
 void tst_QLibrary::version_data()
 {
 #ifdef Q_OS_ANDROID