content: add a test for code snippets

Also: fix bad build tags, mark some snippets as no-build or no-run.

This is a step toward adding the tour to the build dashboard.

Change-Id: I58af8f2289fee4c2ceee3be5a44a3fb986b26296
Reviewed-on: https://go-review.googlesource.com/18555
Reviewed-by: Rob Pike <r@golang.org>
23 files changed