Fix running tests in environment with a custom NINJA_STATUS.

[erikchen]

Don't leak the environment's NINJA_STATUS leak into the tests, which depend on setting a custom NINJA_STATUS.

[dpranke]

Bleah, it turns out this is annoyingly complicated and is going to require a serious re-think (that I'd long been putting off) to fix properly. It may be that this is the right short-term fix, but I'll look at it a bit longer to see if there's a better short-term fix.