Actually, I have had yet another thought on this -- following up
on my earlier suggestion of treating APO as "apart from" in this
context -- which is namely to repunctuate the translation as
Don't have anyone [who is] being tested keep saying that "I'm
being tested apart from God, [just] because God is untestable
by harmful [things or people], while he himself tests no one."
Yet [it is true that] each [person] gets tested [as he keeps]
being dragged away and seduced by his own longings ...
By including the "because" clause in the quotation, the putative
speaker is saying "God is not with me in my testing because
I realize that no one (or nothing) can test him, and he
isn't the author of the testing." Punctuated in this way,
James is saying, yes, the testing occurs when our own longings
get in the way and take over, but that doesn't mean that God
is absent from us in the time of testing.
With this punctuation, too, James is no longer (as Jeffrey notes)
necessarily contradicting the rest of the Biblical witness that
God does frequently "test" people.
This idea follows, of course, from interpreting APO as "apart
from" as opposed to "coming from" (usu. EK/EX) or "by" (usu.
hUPO, and so in v. 14). But James does seem to use APO in this
sense in v. 27. If one interprets APO as either "coming from"
or "by," one gets the traditional interpretations.
Testing, by the way, can be either positive or negative in
English. Engineers refer to both destructive (negative) and
non-destructive (positive) testing. As far as negative testing
is concerned, I know a salesman who wants to be sure his
prospects are trustworthy and reliable, so he takes them out
drinking to see what their character is like when the inhibitions
are down (This, of course, raises all sorts of moral issues,
but that's not my point here). He is not necessarily "tempting"
his prospects, but rather testing them in a situation where
their own desires could take over on them.