TESTING ONLY: Try vim_strnsave.

Travis discovered a memory leak in use of get_isolated_shell_name()'s
use of vim_strsave(). Try vim_strnsave() instead.
Improve unit test.

Add mock_shell() to nvim/misc1.c to set the environment for unit
testing. Expand the unit test to cover multiple setups.

SQUASH WITH "TEST-ONLY: Add unit test for get_..._shell_name()."
fix 'TESTING ONLY: Try vim_strnsave'