provider/null: sporadic test failure

About 1 in 4 test runs, I see the following failure:

FAIL: environ_test.go:102: environSuite.TestEnvironBootstrapStorager

environ_test.go:112:
    c.Assert(err, gc.ErrorMatches, "exit code 99")
... error string = "failed to flush input: write |1: broken pipe"
... regex string = "exit code 99"

OOPS: 0 passed, 1 FAILED
--- FAIL: Test (0.06 seconds)

It looks like the issue is that the fake ssh command is exiting and therefore
we can't write the command to it. That's actually a potential issue
with the real ssh command too - if it fails early, we will only see
"write on closed pipe", not the actual failure,
so I think this isn't just a matter of a bad test.