os/exec: TestCatGoodAndBadFile failures on illumos

[panjf2000]

--- FAIL: TestCatGoodAndBadFile (0.01s)
    exec_test.go:430: expected test code; got "" (len 0)
FAIL
FAIL	os/exec	0.510s
ok  	os/exec/internal/fdtest	0.012s
FAIL
go tool dist: Failed: exit status 1


Error: tests failed: dist test failed: {go_test:os os}: exit status 1

sendfile(3ext) on illumos seems to incur intermittent failures when the target file is the standard stream (stdout, stderr):

https://build.golang.org/log/bb20485d1d72074792215facb7b28ae661e6a6e0
https://build.golang.org/log/2c88f9652638bc186037196f95f84506c2e139c7
https://build.golang.org/log/4de2769138b79cd750b41ea950b148ffb118f6ef

Related CL: https://go.dev/cl/603098

/cc @ianlancetaylor @neild

[gopherbot]

Change https://go.dev/cl/605355 mentions this issue: os: don't use sendfile(3ext) on illumos when target is standard stream

[gopherbot]

Sorry, but I can't find a watchflakes script at the start of the issue description.
See https://go.dev/wiki/Watchflakes for details.

— watchflakes

[panjf2000]

Looks like this flake didn't go away: https://build.golang.org/log/876e7c984d928194c55935da414eeb30e514473f