At the current unoptimized memory utilization of the various data structures,
100k IPv6 routes consumes in the ballpark of 3-4GiB, which risks OOMing our
386 test machine.
Until we have the optimizations to (drastically) reduce that consumption,
skip the test that bloats too much for 32-bit machines.
Signed-off-by: David Anderson <danderson@tailscale.com>