testing: FAIL: TestBRun #15063

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   Tip at 014f3e1 + https://go-review.googlesource.com/#/c/21410/
2. What operating system and processor architecture are you using (go env)?
   linux amd64
3. What did you do?
   Patch https://go-review.googlesource.com/#/c/21410/

go test testing/...

1. What did you expect to see?
   Test passes
2. What did you see instead?

--- FAIL: TestBRun (0.00s)
        sub_test.go:392: MemAllocs was 8; want 2
        sub_test.go:395: MemBytes was 4096; want 512
FAIL

1. Additional information
   First showed up here https://storage.googleapis.com/go-build-log/63567c79/darwin-amd64-10_10_d6336420.log but the with the above patch it can be reproduced on linux amd64, too.