Fix missing original env variable issue on multiple test cases.

[john-yan]

When the parent spawn the child processes, the environment variables passing into the child processes are missing the original env variables passing into the parent. Some missing variables like LD_LIBRARY_PATH cause the child processes unable to run.

PR-URL: #3183

[rvagg]

lgtm

I imagine the proper way to do this is with Object.assign() but since it's tests I'm not sure it matters, anyone else have an opinion on that?

[rvagg]

I'm going to close this one in favour of #3190 which is against master, we'll do the cherry-picking later @john-yan but thanks anyway.