QDoc-ify QTest::runningTest()'s comment

While it's being picked into 6.3, it's not present in 6.3.[01], so
I've left it out of QDoc's sight (even though it's still \internal)
for the version picked to 6.3, but let's include it in internal docs
as "from 6.4" since that's the first minor release to contain it.

Change-Id: I1704a1ca4ba1231d0213e9ca236ef8401a59ddd0
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit b3262c7e540c446c5570c3ebf999adea48b2e2e4)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qtestcase.cpp

@@ -2977,7 +2977,9 @@ bool QTest::currentTestFailed()
     return QTestResult::currentTestFailed();
 }
 
-/*
+/*!
+    \internal
+    \since 6.4
     Returns \c true during the run of the test-function and its set-up.
 
     Used by the \c{QTRY_*} macros and \l QTestEventLoop to check whether to