Merge branch 'jn/test-lint-one-shot-export-to-shell-function'

The test-lint machinery knew to check "VAR=VAL shell_function"
construct, but did not check "VAR= shell_funciton", which has been
corrected.

* jn/test-lint-one-shot-export-to-shell-function:
  fetch test: mark test of "skipping" haves as v0-only
  t/check-non-portable-shell: detect "FOO= shell_func", too
  fetch test: avoid use of "VAR= cmd" with a shell function