Sync is specifically intended for test programs. It is based on an
LLScalarCond<int>. The idea is that each of two coroutines can watch for the
other to get a chance to run, indicated by incrementing the wrapped int and
notifying the wrapped condition_variable. This is less hand-wavy than calling
llcoro::suspend() and hoping that the other routine will have had a chance to
run.

Use Sync in lleventcoro_test.cpp.

Also refactor lleventcoro_test.cpp so that instead of a collection of static
data requiring a clear() call at start of each individual test function, the
relevant data is all part of the test_data struct common to all test
functions. Make the helper coroutine functions members of test_data too.

Introduce llcoro::logname(), a convenience function to log the name of the
currently executing coroutine or "main" if in the thread's main coroutine.