testing: -cpu flag value not recorded in the test output

[davecheney]

This may be a regression from 1.4

go 1.4.2

% ~/go1.4/bin/go test -v -cpu=1,2,4
=== RUN TestWhoop
--- PASS: TestWhoop (0.00s)
        whee_test.go:6: Shoop-da-whoop
=== RUN TestWhoop-2
--- PASS: TestWhoop-2 (0.00s)
        whee_test.go:6: Shoop-da-whoop
=== RUN TestWhoop-4
--- PASS: TestWhoop-4 (0.00s)
        whee_test.go:6: Shoop-da-whoop
PASS
ok      whee    0.005s

go 1.5

lucky(~/src/whee) % go test -v -cpu=1,2,4
=== RUN   TestWhoop
--- PASS: TestWhoop (0.00s)
        whee_test.go:6: Shoop-da-whoop
=== RUN   TestWhoop
--- PASS: TestWhoop (0.00s)
        whee_test.go:6: Shoop-da-whoop
=== RUN   TestWhoop
--- PASS: TestWhoop (0.00s)
        whee_test.go:6: Shoop-da-whoop
PASS
ok      whee    0.003s

[ianlancetaylor]

Intentional change: see #11200 .