proposal: testing: delay registration of flags until MainStart; add Init for explicit registration

[rsc]

Mitchell Hashimoto pointed out at Gophercon that if you write
packages that provides extra test hooks mentioning *testing.T,
the import of "testing" forces the testing flags into any resulting
binary, even non-test binaries. At Hashicorp they define a separate
package that defines a copied testing.T interface only to avoid
these flag definitions.

I sent CL 49251 to delay the registration of the flags so that
those packages can import "testing" directly instead of a fake
package that must be kept in sync.

For discussion for Go 1.10.

[bradfitz]

This happened so enough times in Camlistore that we have a test for it.

I suppose this CL would "break" this code:

https://github.com/camlistore/camlistore/blob/77ed42e/pkg/buildinfo/buildinfo.go#L37-L40

But it wouldn't really matter, since it was only detecting that to avoid the testing flags showing up.

[rsc]

No one has objected, so I suggest we at least try this.

[rasky]

#19533 is a tracking bug for the generic issue of dropping init-time work that is unused. I understand that we're pretty far though.

[rsc]

CL 49251 is approved but needs to be rebased.

[odeke-em]

For the link happy folks, @rsc is referring to https://golang.org/cl/49251

[gopherbot]

Change https://golang.org/cl/49251 mentions this issue: testing: avoid registering flags except in test binaries

[rsc]

Unexpected problem in CL; need to introduce some new API like testing.Init to force registration of flags. Leaving for Go 1.11.