This change is to prevent needed a separate stat syscall for each file
when using readdir.
For consistency, this PR also modifies std's `WalkEntry` interface to
extend `DirEntry` with an additional `path` field.
All Deno runtime test files were moved to cli/js/tests/ directory.
It makes a clear distinction that cli/js/tests/ contains code
that is run under Deno runtime as opposed to code in cli/js/ which
is used to create bundle and snapshot with "deno_typescript".
