Make it easier to use pexpect and to understand its error messages.
Switch to a style in tests using bound methods, which makes them
less noisy to write.
This adds a new interactive test framework based on Python's pexpect. This
is intended to supplant the TCL expect-based tests.
New tests go in `tests/pexpects/`. As a proof-of-concept, the
pipeline.expect test and the (gnarly) bind.expect test are ported to the
new framework.
