Merge branch 'jk/test-lint-forbid-when-finished-in-subshell'

Because "test_when_finished" in our test framework queues the
clean-up tasks to be done in a shell variable, it should not be
used inside a subshell.  Add a mechanism to allow 'bash' to catch
such uses, and fix the ones that were found.

* jk/test-lint-forbid-when-finished-in-subshell:
  test-lib-functions: detect test_when_finished in subshell
  t7800: don't use test_config in a subshell
  test-lib-functions: support "test_config -C <dir> ..."
  t5801: don't use test_when_finished in a subshell
  t7610: don't use test_config in a subshell