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.
