rspec: add coverage to test env var restoration

Commit f447ef521 added a shared context that prevents
environment variables in one example from interfering
with other examples.

This commit adds unit test coverage to prove:

    JumanjimanSpecHelper::EnvironmentContext
      when included
        restores environment variable if changed within example
      when not included
        preserves environment variable if changed within example

Signed-off-by: Paul Morgan <jumanjiman@gmail.com>