internal/web: consistently remove OMIT lines from code blocks

OMIT lines may appear in code before or after the section intended to be
displayed in a code block, however in the case of playground code blocks
those lines are actually included. This change makes sure we always
filter out the OMIT lines when generating code blocks, and adds a test.

I noticed this when the playground snippets in the race detector blog
post (https://go.dev/blog/race-detector) gave this error:

  package play: build constraints exclude all Go files in /tmp/sandbox2684341108

This is because those programs begin with the lines `// +build OMIT`,
which were never intended to be executed.

Change-Id: I5b0320da139741c789126f0c9d3dabc7044cd350
Reviewed-on: https://go-review.googlesource.com/c/website/+/466775
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

0 files changed, 0 insertions, 0 deletions