cmd/go: check for 'svn' binary in PATH in TestAccidentalGitCheckout

This test requires subversion to run, but does not check to see if it's
available before running as it does for git.

Call testenv.MustHaveExecPath to check beforehand to allow the test to
be skipped if the svn binary does not exist.

Change-Id: I16ae104621b221fc6e96f6c7dcd71bf406caa0c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/223082


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>