Test cases where pbkdf2_hmac panics

[briansmith]

Currently there are some test cases in pbkdf2_tests.txt that are commented out because they are invalid inputs for ring's PBKDF2 module. In particular, they request an output longer than the output length of the the digest function being used. We should turn those test cases into the equivalent of [should_panic]. AFAICT, this means we need to:

1. Uncomment those test cases.
2. Annotate those test cases with "FAILS: OUTPUT_LARGER_THAN_DIGEST_OUTPUT".
3. Change the testing code so that for the cases marked "FAILS: OUTPUT_LARGER_THAN_DIGEST_OUTPUT", a task gets spawned that does the test case. We have to wait for the task to end and then verify that it was ended due to a panic!. In other words, we need to create the equivalent of a [should_panic] test.

Additionally, we should add more test cases, particularly a test case for iterations == 0, as BoringSSL has (had) a bug when interations == 0.

AFAICT, this is complicated by rust-lang/rust#25869, as I believe we cannot do [should_panic] tests or create a data-driven alternative to [should_panic] on Win32 since Rust on Win32 doesn't implement panic! correctly; `panic!`` just kills the entire process. We should probably just work around that by not running these tests on Win32.

[briansmith]

#[should_panic] works on all platforms now and is already being used by some tests. So we can definitely do this now.

[joshuata]

I believe this was resolved by f87e2e8 and a316606

[briansmith]

Yes, you're right, given the description above. However, we should add a #[should_panic] test for the case where iterations == 0, which is the remaining case that can panic.

[joshuata]

Okay. I will get on that ASAP.

[briansmith]

Fixed in #466. Thanks again, @joshuata.