[OE-core] [PATCH] local.conf.sample: Add info about -ptest package group

[Maxin B. John]

ptest enabled packages will "inherit ptest". So, we can use grep to
find the list of ptest enabled packages.

eg:
# cd poky
# grep -ir  --include=*.{bb,inc} "inherit"  |grep "ptest"
meta/recipes-extended/bash/bash.inc:inherit autotools gettext update-alternatives ptest
meta/recipes-core/glib-2.0/glib.inc:inherit autotools gettext gtk-doc pkgconfig ptest
meta/recipes-core/dbus/dbus-ptest_1.6.8.bb:inherit autotools pkgconfig gettext ptest
 
Best Regards,
Maxin