travis: add non-default shell testing to travis

This is intended to verify #5038. It should fail in strange ways. With #5038 on top it should be green again.

Rationale: CONFIG_SHELL can be used to tell configure which shell to use. Otherwise it looks for the "best" one, and finds bash on most systems. The value is passed into SHELL inside the make files, which is used for all subshells and tests.

For one test, explicitly ask for dash.