improving the ptest section

  going over the ptest section in the docs:

https://docs.yoctoproject.org/test-manual/ptest.html

and i can see a couple potential improvements but i'll ask a couple
questions first to make sure i did not misunderstand the code and my
testing.

  first, section 3.1 advises that to add ptests to your image, you
need to add those *two* lines. let's back up a second and consider
just the first line:

  DISTRO_FEATURES:append = " ptest"

if you add "ptest" to DISTRO_FEATURES, that means that all of the
ptest-enabled recipes will build and package their "xxx-ptest"
packages and install them under /tmp/deploy/<backend>, correct? but if
you stop there, all that means is that your build will have gone to
all that trouble, only for no ptests to be installed in your image.
and people will think, "why would i do that and then not go on to
install those packages in my image?". which is a perfectly reasonable
question ... unless that setting had been set for you without you
knowing it, which is *exactly* what happens if you build with a DISTRO
of "nodistro", since the defaultsetup.conf file in OE-core includes
default-distrovars.inc, which includes the line:

  DISTRO_FEATURES_DEFAULTS ?= " \
    acl alsa bluetooth debuginfod ext2 ipv4 ipv6 \
    wifi xattr nfs zeroconf pci 3g nfc x11 vfat seccomp pulseaudio \
    gobject-introspection-data ldconfig opengl ptest multiarch wayland vulkan \
    "
which explains why *my* build created all these ptest packages i did
not ask for. so as a first fix, i would explain that to simply *build*
all ptest packages, that feature needs to be set, but it *might*
already be set depending on your build configuration, so it would be
worth mentioning the possibility of *removing* that feature if you
don't want it.

  and second, in addition to simply installing *all* of the ptest
packages, it should be mentioned how to install only *some* of them,
with something like:

  IMAGE_INSTALL:append = " fubar-ptest"

those are two immediate improvements i can see. thoughts?

rday