Change "helful" to "helpful"

The output of helpful--keymaps-containing changed since Emacs 30.x.
This patch adapts its test for newer Emacs versions.

Emacs 30.x added interpreted-function which is different from
byte-code-function, and hence helpful--signature will try to treat it
like a lambda which will fail with

,----
| (wrong-type-argument listp #[(x) (x) (t) nil "Foo"])
`----

This patch adds tests for such case using interpreted-function-p when
it's available.

When `package-version' is not what we expect (e.g a simple string), a `listp`
error is thrown and you are unable to view the variable documentation due to it.
This now handles it and provides a fallback value.