cmd/api: make the test more robust

cmd/api: make the test more robust

Previously, the TestCompareAPI test would fail if runtime.Version()
is "dev", or, more importantly, "go1.5"; because compareAPI depends
on runtime.Version and -allow_new flag. Move that logic out make
its test more robust.

Change-Id: I8f40daa1838b8acd26adac8848762d95315053b0
Reviewed-on: https://go-review.googlesource.com/8622
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>