Add test for with_environment helper
Add test for with_environment helper

We want to use this to enable hooks to set environment variables in
their configuration, so testing seems worthwhile.

oxocode/overcommit,TheSooth/overcommit,TheSooth/overcommit,blech75/overcommit,blech75/overcommit,gnagel/overcommit,oxocode/overcommit,gnagel/overcommit