Add some tests for the default GPS / route-manager handling of
DISCON in flight-plans. Especially check behaviour when sequencing
a DISCON, and resuming with a Direct-to after one.
This requires a corresponding FGData update to the default GPS Nasal.
Ensure changes from Nasal are picked up by delegates (such as GPS) when
they are made. Add a route-manager test which sets and exits a hold
using Nasal.
As part of this, update the test API to make it easy to run Nasal from
a test.
GPS now lets the delegate handle the sequencing behaviour, by
setting a new config property. The default GPS delegate (in Nasal)
now sets this property.
This requires a corresponding FGData update.
