Upcoming flight-planning changes want to use the perf computation code
in route-path, so move it to a public class, and implement some of the
missing functionality, especially correct GS computation for altitude.
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.
