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>