sinit's nil and negative-length tests and stoutf's zero-size test only
had test callers; every real caller passes a buffer and its size. The
len > n check after fullrune succeeded could never fire.
server_test, ibus_test and xim_adapter_test each ran their own copy of
the same alt loop that stands in for imthread, and two of them had
private checkstr/checkenginepreedit variants. test_util.c now provides
Pump (trace, optional hold on a chosen op, stop) in its own proc so a
test may block in socket I/O while the engine runs, and test.h declares
the engine hooks once; every .c includes dat.h and fn.h itself as the
rest of the tree does. server_test's three copies of fixture setup and
teardown became serverbegin/serverend.
