[QTextStream] Add operator bool() to QTextStream class

- Add operator bool() that checks whether the associated stream has
  no error status. This operator makes it possible to use streams
  and functions that return references to streams as loop conditions:
    `while (stream >> text)`.
- Add testcase for testing the new operator.
- Update existing testcases that use or can use the status of stream
  operations.

[ChangeLog][QtCore][QTextStream] Added implicit conversion to bool,
returning `status() == Ok`.

Task-number: QTBUG-52189
Change-Id: I21cbe02ec194078304bcfe606a6f2f572dd09c84
Reviewed-by: Marc Mutz <[email protected]>

src/corelib/serialization/qtextstream.cpp

@@ -2556,6 +2556,13 @@ QTextStream &QTextStream::operator<<(const void *ptr)
     return *this;
 }
 
+/*!
+    \fn QTextStream::operator bool() const
+    \since 6.10
+
+    Returns whether this stream has no errors (status() == \l{OK}).
+*/
+
 namespace Qt {
 
 /*!

src/corelib/serialization/qtextstream.h

@@ -170,6 +170,8 @@ class Q_CORE_EXPORT QTextStream QT_TEXT_STREAM_FINAL : public QIODeviceBase
     QTextStream &operator<<(const char *c);
     QTextStream &operator<<(const void *ptr);
 
+    explicit operator bool() const noexcept { return status() == Ok; }
+
 private:
     Q_DISABLE_COPY(QTextStream)
     friend class QDebugStateSaverPrivate;