Look for broken use of "VAR=VAL shell_func" in test scripts as part
of test-lint.
* es/test-lint-one-shot-export:
t/check-non-portable-shell: detect "FOO=bar shell_func"
t/check-non-portable-shell: make error messages more compact
t/check-non-portable-shell: stop being so polite
t6046/t9833: fix use of "VAR=VAL cmd" with a shell function