cmd/dist: only run swig tests when a go directory is present in swiglib

When there is no go directory inside the swiglib directory then swig
was installed without Go support. Tests in misc/swig will fail when
swig is installed without Go support.

Add additional checks for the presence of a go directory in the directory
reported by 'swig -go -swiglib' to determine if misc/swig tests should
be run.

This avoids all.bash failing when swig but not swig-go is installed
using macports.

Tested on darwin with swig and with and without swig-go installed
using macports.

Fixes #23469

Change-Id: I173201221554982ea0d9f2bea70a3cb85b297cec
Reviewed-on: https://go-review.googlesource.com/88776


Reviewed-by: David Chase <drchase@google.com>