Fail message shouldn't contain answer for hashed tests

[trevorcampbell]

Whenever the test is hashed, we should make sure to avoid the fail message saying things like

"Error: the value is not 8"

because of course that leaks the answer. Should perhaps replace with

"Error: the value is not [hidden answer!]" or something like that.

One twist: for things like type checking, we may want to avoid doing the above. e.g. if someone has the answer

a = np.float64(3)

and the test returns "Fail, the type is not float", that is useful information to help someone answer the question such that the tests pass... then again, maybe testing comparable types is a separate issue...

[trevorcampbell]

more thinking here -- take a look at the example

    int:
        - test: "type({{snippet}})"
          fail: "type is not int"

        - test: "{{snippet}}"
          fail: "value is not correct"

AutoTest will leak the type check for hashed tests (which I find OK) but won't leak the value (also good). So maybe this isn't an issue yet...let's leave this open for now. At least it's something to include in documentation at some point.