This saves us from having to do it every time, and makes things much
nicer. Also use a little bit more specific regex, to avoid issues with
messages that happen to contain the word "at"

Also, fix typo that inverted the meaning of a comment :)

This is to fixes sinonjs#2408, which could result in error messages like
"expected spy to be called 10 times but was called 10 times".

Now we will instead say "expected '10' to be a number, but was of type
string", which is much clearer!

The previous "fix" was wrong, I misunderstood this code and the comment.
What's actually happening here is that we want to add a frame of context
to `callStr`, but the first two stack frames will be within Sinon code
and thus probably not helpful to the end-user.

So, we skip the first two stack frames, and append the third stack
frame, which should contain a meaningful location to the end-user.

This ensures that if at some point we end up with another Sinon layer in
the stack at some point, we'll catch it and hopefully adjust accordingly

For reference, as of this commit, the Sinon portion of the stack is:
lib/sinon/proxy-invoke.js:65:15
lib/sinon/proxy.js:265:26

Also convert a neighboring test to async while we're at it