Adding my tests will succeed.

My tests all run as expected; the original tests that
used to crash silently get dropped.
Initialize to NullTestShell rather than 0.

No functional change/fix; behavior as before.
Works for ordered tests and normal tests before.

('before' with respect to linked list order -
tests would have to occur later in the test file).
Remove knowledge of head_ from UtestShell again.

N.B. Crashes original CppUTestExtTests.
Normal tests before and after ordered tests working.

NOTE: Normal tests inbetween ordered tests will still be ignored (not run).