Remove early return on `startup.cpp` when testing is enabled

Originally startup.cpp caused problems with unit testing and to fix this we added an early return if testing was enabled:

#ifdef REST_TESTING_ENABLED
        return;
#endif

This doesn't seem to affect the running of tests, but makes the framework behave differently when testing is enabled, this means that users should not use the framework with tests compiled to produced results etc. and only use it to run the tests, then compile it again without tests. This is also true for the pipeline where in order to run tests the framework is compiled twice.

There should be a way to modify this file so that tests can be run and also the file keeps working as intended, so that we can avoid recompiling twice.