docs: snap install anbox-cloud-tests requires a channel

[lathiat]

Installing the anbox-cloud-tests channel requires a channel, as no latest/stable release exists. This is not documented.

It appears like the intent is to install a version that matches the deployment version.

$ sudo snap install anbox-cloud-tests
error: snap "anbox-cloud-tests" is not available on stable but is available to install on the
       following channels:

       edge       snap install --edge anbox-cloud-tests

       Please be mindful pre-release channels may include features not completely tested or
       implemented. Get more information with 'snap info anbox-cloud-tests'.

$ sudo snap install anbox-cloud-tests --edge
	error: snap "anbox-cloud-tests" is not available on latest/edge but is available to install on the
       following tracks:

       1.23/edge       snap install --channel=1.23/edge anbox-cloud-tests
       1.22/stable     snap install --channel=1.22 anbox-cloud-tests
       1.22/candidate  snap install --channel=1.22/candidate anbox-cloud-tests
       1.22/beta       snap install --channel=1.22/beta anbox-cloud-tests
       1.22/edge       snap install --channel=1.22/edge anbox-cloud-tests
       1.21/stable     snap install --channel=1.21 anbox-cloud-tests
       1.21/candidate  snap install --channel=1.21/candidate anbox-cloud-tests
       1.21/beta       snap install --channel=1.21/beta anbox-cloud-tests
       1.21/edge       snap install --channel=1.21/edge anbox-cloud-tests
       1.20/edge       snap install --channel=1.20/edge anbox-cloud-tests
       1.19/stable     snap install --channel=1.19 anbox-cloud-tests
       1.19/candidate  snap install --channel=1.19/candidate anbox-cloud-tests
       1.19/beta       snap install --channel=1.19/beta anbox-cloud-tests
       1.19/edge       snap install --channel=1.19/edge anbox-cloud-tests
       1.18/stable     snap install --channel=1.18 anbox-cloud-tests
       1.18/candidate  snap install --channel=1.18/candidate anbox-cloud-tests
       1.18/beta       snap install --channel=1.18/beta anbox-cloud-tests
       1.18/edge       snap install --channel=1.18/edge anbox-cloud-tests
       1.17/stable     snap install --channel=1.17 anbox-cloud-tests
       1.17/candidate  snap install --channel=1.17/candidate anbox-cloud-tests
       1.17/beta       snap install --channel=1.17/beta anbox-cloud-tests
       1.17/edge       snap install --channel=1.17/edge anbox-cloud-tests
       1.16/stable     snap install --channel=1.16 anbox-cloud-tests
       1.16/candidate  snap install --channel=1.16/candidate anbox-cloud-tests
       1.16/beta       snap install --channel=1.16/beta anbox-cloud-tests
       1.16/edge       snap install --channel=1.16/edge anbox-cloud-tests
       1.15/stable     snap install --channel=1.15 anbox-cloud-tests
       1.15/candidate  snap install --channel=1.15/candidate anbox-cloud-tests
       1.15/beta       snap install --channel=1.15/beta anbox-cloud-tests
       1.15/edge       snap install --channel=1.15/edge anbox-cloud-tests
       1.14/stable     snap install --channel=1.14 anbox-cloud-tests
       1.14/candidate  snap install --channel=1.14/candidate anbox-cloud-tests
       1.14/beta       snap install --channel=1.14/beta anbox-cloud-tests
       1.14/edge       snap install --channel=1.14/edge anbox-cloud-tests
       1.13/candidate  snap install --channel=1.13/candidate anbox-cloud-tests
       1.13/beta       snap install --channel=1.13/beta anbox-cloud-tests
       1.13/edge       snap install --channel=1.13/edge anbox-cloud-tests
       1.9/stable      snap install --channel=1.9 anbox-cloud-tests
       1.9/candidate   snap install --channel=1.9/candidate anbox-cloud-tests
       1.9/beta        snap install --channel=1.9/beta anbox-cloud-tests
       1.9/edge        snap install --channel=1.9/edge anbox-cloud-tests

       Please be mindful that different tracks may include different features. Get more information
       with 'snap info anbox-cloud-tests'.

---

Reported from: https://documentation.ubuntu.com/anbox-cloud/en/latest/howto/install/validate-deployment/