unix: use /proc/self/exe instead of os.Args[0] in test.

[hugelgupf]

If these tests happen to be run out of order, TestGetwd can be run before TestPassFD. TestGetwd changes the cwd, so os.Args[0] is potentially not relative to the right directory anymore.

(cc: googlers, this is so I can import a new version of x/sys/unix to g3.)

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Message from Christopher Koch:

Patch Set 1:

(this is me, it was just easier to use the GitHub web ui to open this change.)

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Message from Brad Fitzpatrick:

Patch Set 1:

Seems like this will fail on all Unix without proc self exe.

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Message from Elias Naur:

Patch Set 1:

(1 comment)

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Message from Brad Fitzpatrick:

Patch Set 1:

(1 comment)

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Message from Christopher Koch:

Patch Set 1:

(1 comment)

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