Fix two off-by-one errors with clock vectors

Fix two off-by-one errors with clock vectors

Two errors were identified with managing clock
vectors in the Execution class. The index
assigned to the new transition was previously
`size() + 1` when it should have read `size()`.
Furthermore, the loop for checking "happens-between"
events excluded event 0. Both issues are now resolved
and a test with a race at the beginning was added
as verification