restore the UI test 'debug with two shells' instruction

seems to be working for me now (and since I was the one that commented
the instruction out, seems I must have had something wrong in my
environment)

Change-Id: Ie8e180a22d6a3dec61686cb38fd41b00f22fe5c7
Reviewed-on: https://gerrit.libreoffice.org/65492
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

solenv/gbuild/uitest-failed-default.sh

@@ -20,7 +20,11 @@ where
     Module - the name of the python file (without the .py extension)
     Class - is the name in the "class Class" declaration
 
-Or to do interactive debugging, put a
+Or to do interactive debugging, run two shells with:
+    make debugrun
+    make gb_UITest_DEBUGRUN=T UITest_$2
+
+Failing that, put a
     time.sleep(60)
 in the beginning of the method in the .py file,
 and attach gdb to the running soffice process.