[MERGE] * Fix the test HTTPServer to be isolated from chdir calls made while it is running

[Aaron Bentley]

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Robert Collins wrote:
> Yay for ages spent scratching head. I can't believe this hasn't bitten
> someone yet.

Since this is copy/paste code, I'm okay with merging it as-is.  Perhaps
a comment explaining where this code came from would be nice, but not
necessary.

bb:approve

Aaron
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