virgl: skip test on Debian 9

virgl: skip test on Debian 9

Debian 9 has an older version of mesa, where dri drivers do not link
against glapi explicitly, which causes problems when they are loaded
by newer mesa-native:

pokybuild@debian9-ty-2:~$ ldd -d /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so |grep undefined
undefined symbol: _glapi_tls_Dispatch   (/usr/lib/x86_64-linux-gnu/dri/swrast_dri.so)
undefined symbol: _glapi_tls_Context    (/usr/lib/x86_64-linux-gnu/dri/swrast_dri.so)

compared to Debian 10:

pokybuild@debian10-ty-2:~$ ldd -d /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so|grep undefined
pokybuild@debian10-ty-2:~$

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>