Make wxExecute() unit test work when using Wine too

Use wxTEST_RUNNER in this test, which can be/is set to "wine" by the
GitHub wxMSW CI workflow and can also be set manually if necessary, to
make executing MSW commands under non-MSW systems have a chance to
actually work (but note that the tests still fail when run under Wine).

tests/exec/exec.cpp

@@ -116,7 +116,13 @@ private:
                      wxProcess* callback_ = NULL)
         {
             forceExitLoop = forceExitLoop_;
-            command = command_;
+
+            // Prepend the command with the value of wxTEST_RUNNER if it's
+            // defined to make this test work when using Wine too.
+            if ( wxGetEnv("wxTEST_RUNNER", &command) )
+                command += ' ';
+            command += command_;
+
             flags = flags_;
             callback = callback_;