QTemporaryFile(Name): don't make the path absolute on generation

I need to use QTemporaryFileName in a context where absolute paths are
not allowed because they change the behavior of the system call (the
-at() POSIX system calls); see next commit. This required a fix to a
seemingly unrelated test, which depended on the absolute path, because
QPluginLoader and QLibrary assume a file name with no path components
imply "search the standard places".

[ChangeLog][Important Behavior Changes][QTemporaryFile] This class will
now return relative file paths in fileName() if the file template was
also a relative path (it used to always return an absolute path). The
temporary files are still created in the same directory; this change
only affects the length of the path the function returns.

Change-Id: I79e700614d034281bf55fffd178f65f2b3d602d8
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/io/qtemporaryfile.cpp

@@ -70,9 +70,8 @@ QTemporaryFileName::QTemporaryFileName(const QString &templateName)
         qfilename.append(".XXXXXX"_L1);
 
     // "Nativify" :-)
-    QFileSystemEntry::NativePath filename = QFileSystemEngine::absoluteName(
-            QFileSystemEntry(qfilename, QFileSystemEntry::FromInternalPath()))
-        .nativeFilePath();
+    QFileSystemEntry::NativePath filename =
+            QFileSystemEntry(QDir::cleanPath(qfilename)).nativeFilePath();
 
     // Find mask in native path
     phPos = filename.size();

tests/auto/corelib/plugin/qpluginloader/tst_qpluginloader.cpp

@@ -16,6 +16,8 @@
 #  include <QtCore/private/qmachparser_p.h>
 #endif
 
+using namespace Qt::StringLiterals;
+
 // Helper macros to let us know if some suffixes are valid
 #define bundle_VALID    false
 #define dylib_VALID     false
@@ -128,7 +130,7 @@ static std::unique_ptr<QTemporaryFile> patchElf(const QString &source, ElfPatche
         const char *basename = QTest::currentDataTag();
         if (!basename)
             basename = QTest::currentTestFunction();
-        tmplib.reset(new QTemporaryFile(basename + QString(".XXXXXX" SUFFIX)));
+        tmplib.reset(new QTemporaryFile(QDir::currentPath() + u'/' + basename + u".XXXXXX" SUFFIX ""_s));
         QVERIFY2(tmplib->open(), qPrintable(tmplib->errorString()));
 
         // sanity-check