Merge branch 'ab/lib-subtest'

Merge branch 'ab/lib-subtest'

Updates to the tests in t0000 to test the test framework.

* ab/lib-subtest:
  test-lib tests: get rid of copy/pasted mock test code
  test-lib tests: assert 1 exit code, not non-zero
  test-lib tests: refactor common part of check_sub_test_lib_test*()
  test-lib tests: avoid subshell for "test_cmp" for readability
  test-lib tests: don't provide a description for the sub-tests
  test-lib tests: split up "write and run" into two functions
  test-lib tests: move "run_sub_test" to a new lib-subtest.sh