Re-work how position-init and ATC-manager work together to do
parking assignment and fallback (when the parking is unavailable).
Improve the logic for the reposition case, and teach ATC-manager about
reposition explicitly.
When the parking is unavailable, explicitly fall back to best-runway
selection in finalizePosition.
Add many additional position-init tests, to cover all of this.
Testcase for running posinit twice.
posinit sets various properties which it then reads, so there's
a possibility that it could write a value which causes unexpected
behaviour when run later.
Unclear if this should be a unit test or a system test, but it's
simple enough to be a unit test.
The test_suite/helpers directory has been renamed to test_suite/FGTestApi/ and
all the fgtest namespaces renamed to FGTestApi. The helper functions are now
also placed into either a setUp or tearDown namespace depending on their
function. And a new FGTestApi::PrivateAccessor::FDM namespace has been created
for holding friend classes used to access private or protected variables or
functions from solely within the test suite.
