test: verify shell option internals

This commit adds code coverage to the internals associated with
the child_process shell option. This achieves the following:

- Increased code coverage, which is currently only reported
for Unix.
- Ensures that all six code paths are covered, including the
three Windows variations, and Android which is not tested at
all on CI.

PR-URL: https://github.com/nodejs/node/pull/10924


Reviewed-By: Rich Trott <rtrott@gmail.com>