Tests are grouped.

t/0*: if these fail, bail out and don't waste CPU
t/1*: core components in isolation (marked with module name)
t/2*: synopsis testing - the main use cases advertised in docs
t/3*: non-essential feature testing, experimental features
t/4*: failure mode testing: check warnings, exceptions, and error messages
t/5*: deprecated features (marked with EOL version/date)
t/6+: interoperation with other systems
