arb_texture_storage_multisample: add test for TexStorage*Multisample

V2: Use a legal (sized!) internalformat. Add another test case to ensure
the correct error is generated for unsized formats.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
arb_texture_storage_multisample: add new tex-param test

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>