QPlainTextLogger: FixedBufString: simplify appendf()

qsnprintf() does not make a difference between lvalue and rvalue
arguments, so the wrapper doesn't need to perfectly forward.

Use decay-copy via by-value arguments to potentially reduce the number
of instantiations required for the existing calls.

Adjust indentation so the continuation line doesn't need to be
re-indented when we'll port qsnprintf() to std::snprintf().

Amends 0e8eb20af43ca473b7a7639bb2941c9e6bad537b.

Pick-to: 6.8 6.7 6.5
Change-Id: Idcaa441517fdbf00fefd952db7928731779123ab
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/testlib/qplaintestlogger.cpp

@@ -64,11 +64,11 @@ template <int N> struct FixedBufString
         buf[used] = '\0';
     }
 
-    template <typename... Args> void appendf(const char *format, Args &&... args)
+    template <typename... Args> void appendf(const char *format, Args... args)
     {
         // vsnprintf includes the terminating null
         used += qsnprintf(buf.data() + used, MaxSize - used + 1, format,
-                          std::forward<Args>(args)...);
+                              args...);
     }
 
     template <int Power = 1000> void appendScaled(qreal value, const char *unit)