Rewrite rt interpolation tests

[richfitz]

In #390 I dropped some support functions that were used to set up a couple of tests (same setup in both tests)

• Can interpolate multistrain Rt (test-lancelot-multistrain.R)
• Can interpolate Rt with step changes (test-lancelot-rt.R)

These need a simulation bit written that will work with the removal of the future betas and simulation functions, then make sure that the calculations of beta cope well with the sharp points of the interpolation. The underlying code is tested elsewhere, so this acts mostly as an integration test.