run-make-support: `assert_dirs_are_equal` traverses symlinks

[jieyouxu]

assert_dirs_are_equal will try to read the file content behind symlinks, which is not necessarily the desired behavior. Either we should make it configurable, or avoid traversing symlinks.

[ChrisDenton]

It makes sense to me not to traverse symlinks by default. But some people aren't me.

In any case, either behaviour can be reasonable depending on the test so I'd vote for making it configurable.

[jieyouxu]

Note: this should also have some basic sanity self-test coverage.