Add a function for validating data.tables that might need setDT

[MichaelChirico]

We could eventually have exported function only for testing that as this may be useful for users who are not sure if DT needs to setDT after loading from disk or if it was re-serialized like here.

Originally posted by @jangorecki in #7324 (comment)

[MichaelChirico]

Further context: I saw this in the ?frollapply documentation and think we're better off hiding such implementation details from users:

data.table/man/frollapply.Rd

Lines 87 to 91 in 6cfa8b9

	is.ok = function(x) {
	stopifnot(is.data.table(x))
	out = capture.output(print(attr(x, ".internal.selfref", TRUE)))
	out!="<pointer: (nil)>" && out!="<pointer: 0x0>"
	}

Basically no users should need to encounter ".internal.selfref" besides the package developers.

To replace this usage in ?frollapply, we could export a function like validateDT() that does some quick validity check of a table. A first check could be about .internal.selfref, maybe we could add others over time (e.g. check that it's not malformed, class is data.table|data.frame in the right order, etc.).

[jangorecki]

As for adding other checks we can leave it in scope of #2334.