Add challenge 3 from the verifyThis 2019 challenge

[sakehl]

Add example for solving challenge 3 from the verifyThis 2019 challenge.

Also add the possibility to test with a specific flag. Needed since otherwise the challenge does not verify.

[bobismijnnaam]

Preemptive apologies for not understanding your use case. I have a few tests that use flags already, see here:

vercors/test/main/vct/test/integration/examples/VeyMontExamplesSpec.scala

Line 12 in 7273598

vercors should verify using silicon flags "--veymont-generate-permissions" example s"$wd/leaderelectring.pvl"

Does that work for you? If that is somehow not what you need please ignore my comment :)

[sakehl]

Ohhh thanks a lot, that works! I changed it