drm/i915/selftests: Increase timeout in i915_gem_contexts selftests

Like in the case of several other selftests, generating lots of requests
in a loop takes a bit longer with GuC submission. Increase a timeout in
i915_gem_contexts selftest to take this into account.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727031703.40395-2-matthew.brost@intel.com