test: speed up chan/select5
test: speed up chan/select5

No functional changes.

Generating shorter functions improves compilation time. On my laptop, this test's running time goes from 5.5s to 1.5s; the wall clock time to run all tests goes down 1s. On Raspberry Pi, this CL cuts 50s off the wall clock time to run all tests.

Fixes issue 7503.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/72590045

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang