test/env: multibyte env var to child process

Note: the test fails on non-Windows CI (Travis linux, Quickbuild bsd):
even on master before the env.c changes in this patch-series.
Maybe the unix part of printenv-test.c needs to be revisited.

Signed-off-by: Justin M. Keyes <justinkz@gmail.com>

test/helpers.lua

@@ -751,6 +751,7 @@ local module = {
   hasenv = hasenv,
   hexdump = hexdump,
   intchar2lua = intchar2lua,
+  isCI = isCI,
   map = map,
   matches = matches,
   mergedicts_copy = mergedicts_copy,