LP-1351089 :: Isolate tests from users BASH_ENV

By suppressing bash from loading the BASH_ENV files
Thinking a bit more, IsolationSuite isn't viable in all cases as we rely on $PATH for a bunch of tests (I think?)
Could you please just add a comment here saying why we do this. i.e. to isolate bash invocations from the user's environment, preventing user/environment-specific configuration.