oe-selftest: Error if known problem variables are set
Setting SANITY_TESTED_DISTROS or PRSERV_HOST are known to break oe-selftest.
Rather than have the user experience this, refuse to execute unless the
environment is correct.
Ideally we'd try and unset these but that is a more invasive change and this
at least makes people aware of the problem.
[YOCTO #11292]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>