In the initTestGlobals() helper function, the event manager is added to the subsystem manager (both
created by the FGGlobals ctor). This allows the event manager to be destroyed via the subsystem
manager in the FGGlobals dtor.
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.
