Merge branch 'sg/tests-prereq'

A lazily defined test prerequisite can now be defined in terms of
another lazily defined test prerequisite.

* sg/tests-prereq:
  tests: fix description of 'test_set_prereq'
  tests: make sure nested lazy prereqs work reliably

diff --cc t/t0000-basic.sh
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge