os/exec: exec_test.go fails in AIX

[ayappanec]

--- FAIL: TestStart_twice (0.01s)
    exec_test.go:1856: Start call #1 return err "fork/exec /bin/nonesuch: restart the system call", want "fork/exec /bin/nonesuch: no such file or directory"
FAIL
FAIL	os/exec	1.125s

https://build.golang.org/log/ced5e820c0b9530792c8addab3941a09e9843508

Looks like this happenend after this commit https://go-review.googlesource.com/c/go/+/728642

[mknyszek]

CC @adonovan @neild

[millerresearch]

Test is failing on Plan 9 for the same reason.

Testing for the exact text of the expected error from starting a non-existing command in the TestStart_twice function is too OS-specific. There's already an exception for Windows, but exceptions would also be needed for AIX and Plan 9 and ...?

For most systems, errors.Is(err, fs.ErrNotExist) would match, but AIX seems to be variously returning "restart the system call", "resource temporarily unavailable", and "interrupted system call".

[millerresearch]

In view of the purpose of the test, I suggest the over-specific examination of the first-pass error message isn't really necessary. What really matters is that the second pass reports "already started" and the first pass doesn't.